RECORD: Darwin, C. R. 1839. Observations on the parallel roads of Glen Roy, and of other parts of Lochaber in Scotland, with an attempt to prove that they are of marine origin. [Read 7 February] Philosophical Transactions of the Royal Society 129: 39-81.

REVISION HISTORY: Scanned, OCRed, corrected and edited by John van Wyhe 2002-8, textual corrections by Sue Asscher 12.2006. Re-scanned in colour by J. David Archibald 2010; the plates were scanned from a copy in the collection of Angus Carroll. RN6

IV. Observations on the Parallel Roads of Glen Roy, and of
other parts of Lochaber in Scotland, with an attempt to prove that they
are of marine origin. By CHARLES DARWIN, Esq., M.A. F.R.S.
Sec. G.S.1

Received January 17th,—Read February
7th, 1839.

AFTER the two elaborate memoirs which were read nearly
at the same time, before the Edinburgh Royal Society and the Geological
Society of London, by Sir THOMAS LAUDER DICK2 and Dr. MACCULLOCH,3 on the
parallel roads of Glen Roy4 and the neighbouring valleys, any detailed
account of the physical structure of that remarkable district would be
superfluous. But from the excellence of these papers and the high
authority of their authors, it is necessary carefully to consider the
theories they have advanced,—a necessity I feel the more strongly, from
having been convinced during the few first days of my examination of
the district, that their conclusions were impregnable. Moreover the
results to which I have arrived, if proved, are of so much greater
geological importance than the mere explaining the origin of the roads,
that I must beg to be permitted to enter into the subject in detail.

Section I.—Description of the Shelves.

The parallel roads, shelves, or lines, as they have
been indifferently called, are most plainly developed in Glen Roy. They
extend in lines, absolutely horizontal, along the steep grassy sides of
the mountains, which are covered with a mantle, unusually thick, of
slightly argillaceous alluvium. They consist of narrow terraces, which,
however, are never quite flat like artificial ones, but gently slope
towards the valley, with an average breadth of about sixty feet. There
are only four shelves which are plainly marked for any
considerable length; the lowest one according to MACCULLOCH is 972 feet
above the sea; the next above it is 212 feet higher, and the third,
eighty-two above the second, or 1266* above the sea; the fourth occurs
only in Glen Gluoy; it is twelve feet higher than the third. I shall
refer to them either by their absolute altitude, or as being the upper
or lower one in the part under description, and not as first, second,
or third; for it will be hereafter seen that others occur in every
respect similar, only less plainly developed.

It is admitted by every one, that no other cause,
except water acting for some period on the steep side of the mountains,
could have traced these lines over an exten-

* Some rude measurements which I made
with a mountain barometer lead me to suspect that these altitudes are
at least a hundred feet too great. It is not a point of any importance
with respect to the theory of the origin of the shelves, but I regret
that I did not verify their height with more care.

1 This was Darwin's first full-length scientific paper. In it he argued that the parallel horizontal ledges were marine beaches raised above present day sea level by crustal uplift. He later regarded it as a great blunder on his part when it was shown that they were the shorelines of glacial lakes. See Rudwick 1974 and Herbert 2005.

sive district. The dark line in the accompanying
wood-cut (No. 1.) represents the real profile of a shelf, and is copied
from MACCULLOCH. To this I have added two imaginary lines, of which the
broken one gives the supposed original form of the

1.
2.

A B supposed original surface of rock; C E line of
shelf; C D H line of shelf when expanded into a buttress or terrace.

underlying rock. The formation of the shelf, as may be
here seen, is chiefly due to the accumulation of matter in the form of
a mound, only very slightly projecting beyond the general slope of the
mountain, and partly to the removal or corrosion of the solid rock. The
latter effect, although well marked in some particular spots, cannot
generally be distinguished, and the shelves no doubt are chiefly due to
the accumulation, and not to the removal of matter. In this same
diagram (1.) the covering of alluvium is represented as thicker some
way below the shelf, than at the same distance above it. I believe this
is generally the case, and hence the projection of the shelf is often
very obscure; and when two or three occur, one below the other, their
outline closely approaches to that represented in wood-cut (2.).
MACCULLOCH will scarcely even allow that a shelf in any case forms a
projecting mound; but this certainly is incorrect, and is indeed
contradicted by his own statements, and by that implied in the
comparison of the shelves with the beaches of lakes, which have been
suddenly drained. The shelves entirely disappear, where crossing any
part of the mountains in which the bare rock is exposed; for loose
matter cannot accumulate there, and the rocks themselves from their
laminated structure do not readily become worn into any regular form.
They likewise disappear where crossing any part which is gently
inclined; for their own slope then coincides with that of the alluvial
covering, and cannot be distinguished from it.

The dotted linein the wood-cut
(No. 1.) is supposed to represent the broader terraces, or even plains,
of stratified shingle, sand, and mud, with which the shelves often
become united. These terraces do not differ from the shelves in any one
essential point of structure, but are much broader; and as the matter
of which they are composed is in much larger quantity, a rude kind of
stratification may be generally observed. They occur only where the
bottom of the valley in its gradual ascent rises nearly to the level of
a shelf, or at points on the hill-sides, where it is probable that the
streamlets formerly brought down much detritus to the ancient beaches.

Sir LAUDER DICK has observed* that the shelf infallibly
intersects the head of such terraces or buttresses: this certainly is
the case (as in diagram 1.) with all the smaller ones; and, therefore,
we may infer that their formation dates from the period when the shelf
was a beach. But at the head of the greater valleys, where the supply
of matter must have been more abundant, and where the slope of the land
was highly favourable to its accumulation, the line of shelf sweeps
across and blends into a plain, which has an uniform slope upwards and
downwards above and below that level. Therefore, when the water stood
at any one of the shelves, there were many little deltas which did not
rise above its level, but some greater ones that were continuous with
an upward slope of shingle, filling the bottoms of the main valleys†.

The shelves are chiefly composed of the same kind of
alluvium with that covering the whole surface of the mountain; and they
seem to have been formed, as suggested by MACCULLOCH, by the check
given to the downward descent of ordinary detritus, and that
transported by torrents, at the level of the ancient waters; I could
perceive no difference in the nature of the alluvium above and below
the upper shelf, as stated to be the case by Sir LAUDER‡. It contains
fewer well-rounded pebbles at the greater heights than would have been
expected on any theory of the origin of the shelves; but they are
abundant in the lower and broader parts of the valleys. Nevertheless
where there is any level spot at the height of the upper shelves,
well-rounded pebbles may generally be found, as on the summit of a
rounded hill, or a flat little strait separating some hillock from a
line of shelf (for instance near Craigdhu, on the summit of Meal Roy,
and between Upper and Lower Glen Roy). In these cases the pebbles must
have been almost exclusively formed by the action of the currents and
waves of the former expanse of water. They are frequently derived from
rocks not found in the immediate vicinity: erratic boulders also are
scattered over these mountains. I state these facts distinctly,
because MACCULLOCH says§ that the composition of the alluvium of the
upper shelves is wholly different from that covering the
sides of the broad valleys; whereas the difference is only one of
degree, for which many causes might be assigned.

I have already observed, that the quantity of solid
rock worn away on the line of any shelf is not usually great. At the
narrow entrance, however, of Loch Treig (of which a drawing is given by
Sir LAUDER DICK), on the west side, which is very steep, the gneiss is
worn into smooth concave hollows, the peculiar curves of which, though

† These statements are founded on what I
saw in Glen Collarig, where the lower shelf (the 972 feet one) blends
into a slope, now rendered irregular by the action of the torrents,
which rises (at the gap) to a height of more than a hundred feet above
the level of the shelf. Again, near the head of Lower Glen Roy, the
seam shelf blends into a similar kind of plain, which rises (at the
base of a terrace, projecting from the next shelf to it,) ninety feet
(barometrically measured) above the level of that shelf to which it may
be said to belong. In the east arm of Glen Turet, the upper shelves in
a like manner terminate in slopes, which rise above their proper levels.

they cannot be described, may be readily imagined by
calling to mind the form of rocks washed by a water-fall. This was the
only one spot where I could observe this appearance in an unequivocal
manner; but this one point of rock would to my mind carry demonstration
with it, even if there were not innumerable other proofs, that the
water had remained at the level of the 972 feet shelf for a very long
period*. On the opposite side of the entrance, or gorge, which here
slightly bends before entering Loch Treig, the shelf expands into a
line of terrace. Standing on the precipitous and waterworn rocks, it
required little imagination to go back to former ages, and to behold
the water eddying and splashing against the steep rocks on one side of
the channel, whilst on the other it was flowing quietly over a shelving
spit of sand and gravel. The only other and rather different case of
waterworn rock, which I noticed, was at the head of Lower Glen Roy
(pointed out by Sir LAUDER DICK), where the summits of some irregular
hummocks of gneiss on a level with the upper shelf were obliquely
truncated by a smooth surface. I have frequently observed a similar
structure on the rocky shores of protected harbours. Large fragments of
rock are scattered on most of the shelves, of which many are of
granite, and have come from a distance, as will presently be described;
the greater number, however, have merely rolled down from the heights
above. Of the latter, some have fallen recently, whilst others are
waterworn, as if they had lain for centuries on a sea coast; and it was
in many cases easy to point out, whilst walking along the level shelf,
which fragments had been washed by the ancient waves, and which had
fallen since.

Sir LAUDER DICK has observed, and the fact is very
important, that the head of Glen Gluoy is separated from the head of a
branch of Glen Roy by a flat land-strait, with which the shelf in the
former glen is exactly on a level; so that if Glen Gluoy were filled
with water to the full level of its shelf, or a few inches
above it, besides a great barrier at the lower end, a little mound,
perhaps a foot or two in height, would be required to prevent the water
flowing into Glen Roy. In the same manner if Glen Roy were closed at
its lower end, and if water stood at the level of the upper shelf, it
would trickle into the valley of the Spey. The same thing would happen
with the lower shelf at the head of the valley of the Spean; and
lastly, a short shelf, which I discovered in a gully, which enters the
Caledonian Canal near Kilfinnin†, between

* After the elaborate arguments given by
MACCULLOCH, to show that no sudden rush of water, or debacle, could
have formed the shelves, I should not have offered any remarks on this
point, had not so distinguished a person as Sir GEORGE MACKENZIE
(London and Edinburgh Philosophical Magazine, December 1835,)1 suggested
such an hypothesis, without, however, it is fair to add, having visited
the district. Each of the ten thousand pebbles, which together form any
one buttress or little delta, and which, it is evident, were
accumulated by the action of one streamlet, at the spot where it
entered the expanse of ancient water,—each of these pebbles required
time for its attrition,—each now plainly speaks against such an
hypothesis.

† I was informed, but whether correctly
I do not know, that the hamlet (in the middle of which there is a mound
with a round tower on it,) on the opposite side of the valley, and a
mile or two south of Invergarry, was named Kilfinnin. I therefore shall
denominate the small stream which flows towards the Caledonian Canal at
that point by this name. In the same manner I shall call the larger
stream which debouches by Habercalder, and its valley, by that name,
not having been able to learn any more proper one.

Loch Oich and Loch Lochy, is in a similar manner on a
line with a peat moss, forming the watershed between it and another
small valley. These four cases are so remarkable, that the coincidence
of level must be intimately connected with the origin of the shelves;
although such relation is not absolutely necessary, in as much as the
middle shelf of Glen Roy is not on a level with any watershed. Sir
LAUDER endeavours to explain this fact by supposing that when the
imaginary barriers of his separate lakes were perfect, the water flowed
from that end of the glen, which is now highest, in other words, that
the drainage of the supposed lakes was in each case in a reverse
direction to that of the streams now occupying their beds. This view
implies, moreover, the strange accident, that, during the breaking down
of the barriers, the part that was originally lowest always remained
standing, whilst a higher part gave way; and thus the removal of the
barrier must be supposed to have happened from the effects of some
causes no ways analogous to the wearing down of the mouths of lakes as
they ordinarily exist.

The structure of these land-straits must be now
described. This has already been minutely done by Sir LAUDER with
respect to that one which connects the sources of Glen Gluoy with those
of Glen Turet, one of the arms of Glen Roy. The only additional
observation which I have to make, is that the strait is broad and very
level, and that on one side I noticed a beach, like that on a sea
shore, of well-rounded pebbles. The accounts given by MACCULLOCH and
Sir LAUDER of the division of the waters of Glen Roy and the Spey
differ in some essential points. The latter author states that the
upper shelf of Glen Roy is on a level (excluding the peat-moss) with
the flat where the waters divide. This appears to be accurately the
case, as far as the mountain barometer (which stood at the same
thousandth of an inch on the two stations) and my eye could be trusted.
But on the north side of the watershed there are patches of little
terraces about fifteen feet above this level, resembling those which in
other parts are connected with shelves, and hence probably having a
similar origin with them. On the hill-side higher up, other obscure
patches of alluvium occur with somewhat analogous forms. The water of
the Spey first flows down a gentle mossy slope eastward, and is then
collected in Loch Spey. On the south side of this loch, there is an
obscure line of terrace, which appears to be about sixty feet above
the loch, and which doubtless led MACCULLOCH to suppose the upper shelf
of Glen Roy was that number of feet above the division of water. The
terrace above Loch Spey, as far as I could judge by the eye without a
levelling instrument, is horizontal, and may perhaps be traced along
the south side of the watershed, even a short distance within Upper
Glen Roy, where certainly there occurs a mound parallel to and above
the upper shelf. I much regret I was unavoidably prevented from
examining this locality with all the attention it deserved. But from
the structure of the small terraces, it appeared to me certain that
water must for a period have occupied a level above that of the highest
shelf of Glen Roy; and likewise that fragments of a shelf, or line of
terrace, which as far as the eye could judge was horizontal, extended
within

the basin of the Spey, and therefore beyond the limits
of the supposed lake of Roy. This latter fact is, at least, certain,
for I have since learned, through the kindness of Sir DAVID BREWSTER,1 that he has seen, as will be hereafter mentioned, shelves resembling
those of Glen Roy at two points, at a distance of several miles down
the valley of the Spey. The watershed at the head of the valley of
Kilfinnin, has precisely the same character with the foregoing cases:
here also a flat-topped buttress projects on one side above the level
of the shelf, and this seems to indicate, as in the former case, the
presence of water at a level rather above that of the shelf itself*.

The division of the waters between most of the glens
and ravines in this district, in situations where no shelves occur,
does not take place on a sharp ridge, but on level, and often broad
land-straits, similar to those just described. I may instance a long
one (at an elevation of between 1400 and 1500 feet above the sea,)
separating two branches of the water which flows by Habercalder into
the Great Glen, and one branch of the Tarf Water. Again another one
nearer Fort Augustus, separating the two lowest and nearest branches of
the same two rivers; here also there were obscure buttresses on each
side above the level of the watershed. An intelligent shepherd who
accompanied me, remarked that this form of the land was common wherever
the waters in this mountainous country divided; and I observed several
instances of it. Finally, I may remark, without wishing to lay any
great stress on the argument, that these land-straits,
whether connected with the shelves, or not, are precisely what might be
expected from straits, properly so called, between arms of
the sea being laid dry.

The discovery of any shelf, beyond the limits where
they had been hitherto observed, being evidently an important point
with regard to the theory of their origin, I shall fully describe the
following case. At the head of a small stream which joins the
Caledonian Canal, near Kilfinnin, and which is divided from the waters
of the Habercalder by a flat mossy watershed already alluded to, some
fragments of a shelf occur on the northern side. This shelf resembles
in every respect those in Glen Roy; it seemed, as I walked along it,
perfectly level, as it likewise did, when I viewed it from either end,
and when I crossed the valley. I then took several measurements at the
most distant points with the mountain barometer, and the mercury stood
within the same hundredth of an inch. On the northern side of the
valley, the shelf, which commences on a level with the mossy plain
dividing the waters, extends for about a quarter of a mile almost
continuously; it is then lost from the number of fragments of rock
which have fallen down the hill, but reappears at the distance of more
than half a mile from its commencement under the form of two or three
little buttresses. These I ascertained by the barometer to be on a
perfect level with the commencement of the shelf, or the watershed, a
circumstance which was also apparent by the eye alone. The line further
on disappears from the rockiness of the sides of the valley.

* The pass of Muckul, described by Sir
LAUDER, which separates the waters of the Spean from a branch of the
Spey, I did not visit.

1 Sir David Brewster (1781-1868), Scottish physicist who specialized in optics. He invented the kaleidoscope in 1816.

On the south and opposite side of the valley, a broad
sloping terrace extends at a corresponding level for about three
quarters of a mile, but is indistinct owing to the gentle slope of the
mountain. Further on it seems modelled into more than one terrace: and
these, though obscure, appear to a person standing on them perfectly
horizontal. Although the terraces are not plainly developed on this
side, yet it is certain, that horizontal mounds, at nearly the level of
the watershed, extend about two miles on the face of the mountain. With
respect to the absolute elevation of this shelf, I made it about forty
feet above the upper one of Glen Roy, and 1120 above Loch Lochy, or
1202 above the sea: but my barometrical observations have no
pretensions to accuracy. After having observed this shelf from so many
points of view, I am prepared positively to assert that it is in every
respect as characteristic a shelf as any in Glen Roy; and although the
fragments of it do not extend over more than, perhaps, half a mile in
length, its origin must be as carefully attended to in any general
theory of the formation of the shelves, as if its length had been
twenty times as great. Its want of continuity and shortness possess,
indeed, in themselves much interest, because we thus know that those
causes which have marked with horizontal lines the sides of the
mountains of Glen Roy in so wonderful a manner, have been in action
here, though they have produced but little effect. Moreover, we see
that if the surface had been originally rather more rocky, or had been
less steeply inclined than at present, or had been subjected to a very
little more alluvial action, all evidence would have been obliterated
of the extension thus far of the action of these causes.

I have already alluded to the important fact
communicated to me by Sir DAVID BREWSTER, namely, that he has seen
shelves in the valley of the Spey. At Phones, which is situated about a
mile from the Truim, and about five above its confluence with the Spey,
one broad and well-marked shelf occurs, along which a carriage can be
driven. On the banks of the Spey, about twenty-five miles below its
source, two shelves occur in an elevated angle between the Burns of
Belleville and the river. They are small; the upper one, however, is
very broad; and their elevation is about 800 feet above the sea. Sir
DAVID BREWSTER says, that the shelves in both places appear horizontal,
and that they resemble those of Glen Roy, though possessing far less
grandeur and symmetry. The fact of their occurrence at these distant
points is, as we shall hereafter see, highly important.

Sir LAUDER believes that a separate lake existed in
each valley, where we now see a shelf, and was separately drained. In
Glen Roy, where three shelves occur, all plainly developed, (with the
exception of Belleville, this is the only place where more than one has
been observed,) the arguments in favour of a separate lake possess the
greatest force. Without entering into any description of the physical
features of Glen Roy, inspection of the accompanying map, taken with
some few alterations

from that of Sir LAUDER DICK, will show the course of
the shelves; although they cannot of course be followed nearly so
continuously in nature as here represented. The lower one (972 feet
above the sea) is common to nearly the whole line of the Spean and Glen
Roy. The two upper shelves are confined to Glen Roy, with the exception
of those short portions extending into Glen Collarig. It will be seen
that both these lines, if continued round the hill of Bohuntine (at the
eastern entrance of Glen Roy), would insulate it, whilst the lower
shelf only forms it into a peninsula. From this structure it will be
evident, that in order to form Glen Roy into a lake at either of the
two upper levels, it would be necessary to erect two barriers, one
across Glen Collarig, and the other principal one across the mouth of
the Roy.

The lines are here represented as if abruptly cut off,
but this is not so; and the following remark holds good in other
cases, namely, that where a shelf terminates without any visible change
in the nature of the slope, such as being rocky, &c., its
disappearance is so extremely gradual, that it can be traced, sometimes
to a further and sometimes to a lesser distance, according to the point
from which it is viewed. Of this fact the shelves on the south-east
side of Glen Collarig offer an excellent example. In the map, the
extremities of the lower of the two upper shelves are represented at
the four places where they terminate, as extending beyond those of the
upper one. I state this on the authority of Sir LAUDER DICK with
respect to those in Glen Roy, and it is conspicuously the case with
that pair in Glen Collarig which I have described as disappearing in so
insensible a manner. The lower line can there be traced, though
faintly, to a point below the houses of the glen opposite a small
tributary torrent, and therefore considerably beyond (or nearer the
mouth) than the point where the 972 feet shelf crosses the bottom of
the valley. Observing in Glen Collarig the gradual disappearance of
either set of lines, and that there is not the smallest apparent cause
for it in the nature of the ground, the first and obvious supposition
is that a sheet of water extended from the Spean into Glen Roy and
Collarig, and that the mere widening of the mouths of the latter, as
they approached the less protected expanse of the Spean, gradually
became unfavourable to the accumulation of detritus, and therefore to
the formation of the shelves. This view is greatly strengthened by the
extension of the lower line in each case beyond the upper; for of
course the supposed unfavourable condition for their formation, that
is, the too great breadth and exposure of the sheet of water of which
they formed the beach, would affect the line when the water stood at
the higher level to a greater distance from the main expanse, or
further up the valley, than when it occupied a lower level. It may,
however, be argued (and on the hypothesis of Glen Roy having existed as
a lake it must be so argued), that as the higher line is the oldest, so
its terminal portion may soonest have yielded to those causes which
modify the surface of the land. This view, however, receives little
support from an examination of the rest of the glen, inasmuch as the
two shelves through its whole course are in a state of equal
preservation. We must therefore conclude, either that we now behold the
shelves

precisely as they were left by the sheet of water, or
that if the two upper shelves did originally extend for an equal length
on each side of the two glens, that the causes which tend in a small
degree (for the existence of the shelves proves that no great changes
have taken place) to smooth the surface, have acted over this district
with the most perfect uniformity. Moreover, it may be
remarked, that wherever a streamlet crosses a shelf, and it is probable
from its size that it formerly delivered detritus to the ancient
expanse of water, either a greater breadth of shelf or a small buttress
there, attests that it was so; and in doing this, likewise attests how
perfectly the surface of the land has been preserved. Now I paid
particular attention to the following observation, namely, that on both
sides of the hill of Bohuntine, and on the opposed mountains, where the
shelves terminate, there was not the smallest change in the composition
or in the outline of the smooth rounded surfaces. Yet it is in this
very spot, where the lines insensibly disappear,—on these very hills,
where the little deltas of the ancient streamlets are still
preserved,—within this very district, where in the extension of the
lower shelf beyond the upper one in the four cases, we have the most
satisfactory proof of the action of absolutely uniform causes, either
in their formation or in their obliteration; it is here, where the
slope of the turf-covered hills is unbroken, where there is not a
remnant of any projecting mass, that we are compelled by the theory to
believe that the two enormous barriers stood, which formed Glen Roy
into the imaginary Loch Roy.

But as it is highly important to show that such a Loch
could not have existed, we must for a time, in the face of these great
difficulties, suppose the two barriers to have been erected. It may be
first remarked, that from the extension of the middle shelf, the
barrier in Glen Collarig could not have occupied the only one place,
which the structure of the ground indicates, even in the smallest
degree, as probable, namely, at the Gap, where the waters divide; but
it is necessary to suppose that it crossed the glen at a point some way
distant from the Gap, and where the valley has a depth, below the upper
shelf, of more than 300 feet. Glen Roy being now converted into a lake,
with its drainage reversed, that is, with the water flowing from it by
the Spey to the east coast of Scotland, let one of the two barriers, we
will say the smaller one in Glen Collarig, give way from the effects of
an earthquake or other cause. The lake will now stand at the level of
the middle shelf, the barrier having given way eighty-two feet
vertically. Again let it burst, and this time rather more than 212 feet
vertical must be swept away, so that the larger lake, supposed by Sir
LAUDER'S hypothesis to occupy the valley of the Spean at the level of
the 972 feet shelf, might send an arm a little way up the glen (as
shown by the shelf now existing there) above the point where the
barrier stood. Let all this have taken place, but still a barrier
nearly a mile long, and 800 feet in height, is left standing across the
mouth of the Roy. Must we suppose that each time the barrier in Glen
Collarig failed, the one in Glen Roy gave way the same number of
feet through some strange coincidence? or are we to conclude that
some awful catastrophe at sub-

sequent times, unconnected with the drainage of the
lake, which must have passed through the breach already opened, removed
the second barrier (either part or all of it) when above water,
without having left the smallest remnant of it, or having disturbed the
smooth alluvial covering of the steep slopes? The 972-feet shelf is
common to the valley of the Spean and Glen Roy, and is supposed to have
been formed by a lake, the barrier of which, some miles in length,
extended near Highbridge across the mouth of the Spean. This shelf
passes uninterruptedly, and with its usual breadth, on both sides of
Glen Roy and of Glen Collarig, in the very part where the barriers of
Loch Roy, if they existed, must have crossed the valley;
therefore the whole, or part of the great base of those enormous
barriers, must have been swept away when submerged within the bosom of
the imaginary Loch Spean; and this must have been so perfectly
effected, that no trace of them is left on the smooth slope of the
hill, not even by a greater breadth of the shelf, any more than in the
part of the second barrier, which must have been removed when above
water*. And all this is supposed to have taken place on the hills,
where I have shown how wonderfully the features of the land have been
preserved, and where the boulders which were washed by the waves of the
ancient water can be distinguished from those which have fallen since.
In conclusion, therefore, I do not hesitate to affirm, that more
convincing proofs of the non-existence of the imaginary Loch Roy could
scarcely have been invented, with full play given to the imagination,
than those which are marked in legible characters on the face of these
hills†.

The same reasons which render the existence of a
separate lake in Glen Roy so excessively improbable, apply with only
little less force to each of the imaginary lakes in the other glens. We
are, therefore, in giving up Loch Roy, involuntarily driven
to the theory advanced by MACCULLOCH, namely, that all the valleys in
which shelves occur were included in one large lake; but we shall thus
run headlong even into greater difficulties. First, from the structure
of the mountains, four immense barriers are required to form the lake‡,
namely, one low down across the valley of the

* I have not thought it worth while to
enter into all the possible cases of this hypothesis, but have merely
taken the most obvious one, which was assumed by Sir LAUDER. If any one
has the boldness to come forward from the obscurity of past times, and
state his belief that the broad barrier of the Spean was erected as
well as removed altogether subsequently to the removal of the two
barriers of Glen Roy, then the objection from the uniform breadth of
the 972-feet shelf, where crossing the spot which must have been
occupied by the barrier of Loch Roy, has less weight, but the other
part of the argument remains valid. Again, on the hypothesis in the
text, I have not entered into all the possible alternatives of the
manner in which the bases of the Loch Roy barriers might have been
removed, either when Loch Roy itself, or when Loch Spean was drained,
or at some subsequent period by unknown causes connected with the
drainage of the imaginary lakes.

† It should be remembered that it is far
easier to assert than to disprove. If to explain some phenomenon it was
stated that the Thames near London was formerly crossed by a barrier
some hundred feet in height, of which it was not pretended a vestige
now remained, it is difficult to imagine what kind of evidence would be
sufficient to prove the hypothesis false, as long as any one was found
willing to admit such an assumption.

‡ I may add, the same number of barriers
are requisite, whether we suppose the existence of one, two, three,

Spey, two at distant points across the Great Glen of
Scotland, and a fourth across the mouth of Loch Eil, the last being
necessary, as MACCULLOCH shows*, from the structure of the Great Glen
in that part. It may be safely asserted that more improbable situations
could hardly be imagined in the whole of Scotland. It is perhaps
useless to ask, were the barriers composed of rock or alluvium? if of
the former, they were transverse to every line of hill in this part of
the country; if of alluvium, we must assume an unexampled case; for
where in the whole world shall we find even one barrier a mile and
upward in length, and 1200 feet high, composed of loose waterworn
materials? Secondly, the theory of one large lake does not explain in a
satisfactory manner the remarkable coincidence between the shelves and
the watersheds. Thirdly, when by the bursting of any one of the
barriers, the level of the lake had fallen from one shelf to another,
the hypothesis requires (as with Loch Roy) that the three
other
barriers, now high and dry, and distant many leagues from each other,
should have been swept away by some unknown power, acting by some
unknown and scarcely conceivable means, from the smooth sides of the
mountains, without a remnant of them having been left; so that
MACCULLOCH even frankly confesses one part is almost as probable (I
would say improbable) as another for the position of the barriers. And
it should be borne in mind, that these extraordinary forces are
supposed to have acted on the outskirts of that large area, throughout
which we have proofs, most wonderful and unequivocal, of the entire
preservation of the surface of the land, as it was left at a period
long anterior to the removal (if such removal ever did take place) of
the barriers of the lower lakes. I do not hesitate to assert that this
one difficulty, even by itself, would be sufficient to refute the
theory of one great lake: Sir LAUDER'S theory has been shown to be
equally untenable. It is perhaps here almost superfluous to add, that
the discovery of the shelf at Kilfinnin (and probably likewise of those
in the valley of the Spey) increases every difficulty manifold; for the
valley of Kilfinnin is almost as wide as it is long, which affects one
theory, as the lowness of the opposite side of the Great Glen does
equally the other. Finally, then, in giving up both, the conclusion is
inevitable, that no hypothesis founded on the supposed existence of a
sheet of water confined by barriers, that is, a lake, can be
admitted as solving the problematical origin of the "parallel roads of
Lochaber."

Section III.—Proofs of the retreat of a body of
water from the central parts of Scotland, and that this water was that
of the sea.

Having now discussed these views which cannot be
admitted,—a method of reasoning always most unsatisfactory, but
necessary in this instance from the high authority of those who have
advanced them,—I will consider some other appearances,

or as many lakes as glens; and the
argument against MACCULLOCH's hypothesis of one lake, and against that
of the separate lakes by Sir LAUDER, are applicable to any hypothesis
requiring an intermediate number.

which will perhaps throw light on the origin of the
shelves. The valley of the Spean, from the point where it joins the
Great Glen of Scotland to where it receives the Roy, is broad, and its
bottom moderately level. The solid rock is concealed in almost every
part, excepting where the river has cut itself a gorge, by irregularly
horizontal strata of gravel, sand, and mud. Large portions of these
beds have been removed along the centre of the valley, yet it is quite
evident from the fringe or line of terraces which skirt each side, that
the bottom must originally have formed a smooth concave surface
inclined towards the mouth of the valley. Portions more or less perfect
of this same deposit can be followed up the course of the Roy, and up
the higher parts of the Spean, where the valley is not too rocky or
narrow, to near Loch Laggan. This loch is but little below the 972-feet
shelf; and at present I wish, for the sake of the independence of the
argument derived from the facts to be stated, to consider only that
part of the country which is below the level of that shelf. These
irregularly stratified beds, near the mouth of the Spean, attain a
thickness of several hundred feet, and they consist of sand and
pebbles, many of the latter being perfectly waterworn. Higher up the
valley, near the bridge of Roy, the thickness before the central
portions were removed appears to have been about sixty feet, but of
course the thickness varies according to the original irregularities of
the rocky bottom of the valley. Now it may be asked by what agency has
this sloping sheet of waterworn materials been deposited along the
course of the valley? From the presence of the horizontal shelves we
know that there has been no change in the relative level or inclination
of the country since this district was last covered with water, and
therefore we may argue with safety, that the action of the rivers, as
far as it is determined by their inclination, must have been the same
since that period as it now is, with the exception of that amount of
change which they may have effected in their own beds. Our knowledge
that there has been here no axis of elevation, with one part always
rising a foot, and another a few inches less; but that the entire
system of drainage has remained undisturbed and subject only to its own
laws of change, is a circumstance which gives a singular degree of
interest to the examination of this district. Now if we look at any
portion of these rivers, for instance the Roy above its junction with
the Spean, we find it has cut a narrow steep-sided gorge through the
solid rock, which is in many parts between twenty and thirty feet deep,
whilst on each side there are remnants, as above stated, of a
continuous bed of gravel, at least sixty feet in thickness. These beds
have certainly been deposited by rapid currents of water, but not by
any overwhelming debacle, as may be inferred from the presence of cross
layers, and the alternate ones of fine and coarse matter. Seeing also
the evident relation of dimension and materials which exists between
these deposits and the valleys in which they occur, it can scarcely be
doubted that the detritus of which they are composed was transported by
the existing rivers. But are we to suppose that the river, as in the
case of the Roy, first deposited along its whole course these layers
one over another, thus raising its bed sixty feet above the solid rock,
and then

suddenly commenced, without the smallest change in the
inclination of the country, not only to remove the matter before
deposited, but when having gained its former level, to act in a
directly opposite manner, and to cut a deep channel in the living rock?
Assuredly such a supposition will not be received; and whatever part
the river had in the accumulation of these waterworn materials, from
the very moment (neglecting the annual oscillations of action from the
changing seasons) it ceased to add and began to remove, its power must
have undergone some most important modification.

It will perhaps be thought that the mere deepening of
the bed of the stream, near the mouth of the valley (the effect being
slowly propagated upwards), could have caused the difference between
the present and the former action of the river. But it is not difficult
to replace in imagination the solid rock in the course of the Spean;
and although a few small lakes will be thus formed, the average slope
will not differ greatly from the present inclination, and this
inclination we see is sufficient to cause the river to wear a deep
gorge in the solid rock, and therefore it is evident (although I am
aware that without actual measurement of the inclination this argument
must rest upon eyesight, which cannot generally be trusted) that a
change of this nature would be wholly insufficient to reverse the
action of the river, as has here been the case. We must not, of course,
at the same time replace in imagination those unconsolidated deposits,
the origin of which we are considering; otherwise no doubt the
inclination of the bed of the river would be greatly altered; although
even in that case I by no means believe that the river would be so much
retarded as to deposit matter at the heights where it is now left. Some
check, therefore, to the transporting power of the stream seems to have
acted at many, or at every successive level. If we reflect on what
would result, as an hypothesis, from a river delivering during a long
period detritus into a lake, the level of which was gradually sinking
from the wearing down of its mouth, a gently sloping surface would be
formed at its head. But as the barrier was cut deeper and deeper, and
the lake sank, the stream in the part where it was once
checked by meeting with the still water would gain velocity, and hence
would cut through the beds which it had originally deposited. The
fringe, of rudely stratified alluvium, the origin of which we are
considering, resembles both in structure and composition such beds of
detritus as would have accumulated on the shores of a lake, had one
existed in these valleys. If, then, we suppose that a subsiding sheet
of water did actually fill this valley, either of one or more lakes,
with their barrier gradually wearing down, or of an arm of the sea, the
general level of the ocean being stationary during a slow elevation of
the land (as now is the case with the fiords of Scandinavia), every
appearance on the sides of the valley of the Spean and Roy will be
explained; and as there is no other way, that I can see, of accounting
for them, the hypothesis is so far worthy of admission.

I ought, perhaps, to have previously observed, that
these deposits could not have been formed when the valley was filled
with water to the level of the shelves, for the

detritus has the character of matter accumulated in
shoal water, and the beds abut abruptly against the bases of the
mountains, instead of blending with the alluvium on their surface, as
would necessarily have happened had the whole been deposited at the
same time at the bottom of one basin.

The conclusion, that these valleys have been occupied
by a sheet of subsiding water, follows more plainly from a somewhat
different class of facts. I have before remarked, that where a
streamlet crosses a shelf, especially if it be the lower one, an
obliquely truncated buttress, the form of which was represented by
dotted lines in the wood-cut No. 1, projects from the side of the hill.
It is quite evident that these were accumulated when the shelf existed
as a beach, and the streamlet at present only acts in removing those
portions with which it comes in contact. Now in some points where the
buttresses have been somewhat largely developed, smaller ones at a
lower level, composed of the same irregularly stratified waterworn
materials, having nearly the same outline, although unconnected with
any shelf, may be observed adhering to the slope of the hill. Instances
of this structure occur on the east side of Glen Roy; on the south side
of the Spean, and between Loch Treig and the bridge of Roy, the
accumulation of perfectly rounded shingle, like that on a sea-beach,
was enormous.

3.

The internal structure in this instance corresponded to
the external form, as is shown in the accompanying diagram, where
highly inclined beds of sand and coarse gravel are capped by other
irregular ones of the same composition, only slightly inclined. In all
these
cases, where the flat-topped buttresses occur on steep slopes, it is
certain (as might have been expected) that the streamlet is steadily at
work in removing matter, and does not add one pebble to the mound. No
one will dispute, that those buttresses, which are mere extensions of a
line of shelf, were formed at the edge of an expanse of water (of which
the shelf was the beach), and it is therefore by itself probable that
the other buttresses, of similar external form and composition, though
occurring at a different level, had a similar origin. But the argument
may be put in a stronger point of view: taking the course of one of
these streamlets, and observing the size and position relative to it of
the buttresses one above the other, it becomes evident that the
materials of which they are formed were accumulated through the agency
of this stream, although it is at the same time inconceivable that they
were left (especially in such a case as that represented in diagram 3.)
on the steep slope by a power which, as it now acts, is steadily at
work, tearing away matter in its whole downward course. Therefore, it
is absolutely necessary to bring into play some intervening or
modifying cause in the action of the streamlet; in the case of the
buttresses which are connected with the shelves, no one can doubt what
this intervening cause has been; shall we, then, rejecting a vera
causa, seek some other one, if indeed such other can be found? Cer-

tainly not; and the conclusion is inevitable, namely,
that a sheet of water must have stood at as many levels as there are
buttresses, and this will include by short steps the whole space
between the bottom of the valley and the lower shelf. Judging also from
the amount of matter accumulated, we must infer that the water remained
at these levels for no inconsiderable periods, although for a lesser
time at each than at the level of the 972-feet shelf.

I would even further add, that in any valley (the
relative level of the country, one part with another, having remained
constant) a single buttress, if composed of such materials as could not
have slided down the face of the hill in mass, or could not, judging by
the presence of cross layers and alternations of fine and coarse beds,
have been deposited by a debacle, indicates that the valley was once
partly or entirely filled up to that height by such matter; and if the
mass be too thick, or at too great an elevation on the sides of the
valley, to allow of the supposition that it was deposited by the
streams now flowing in the valley, subject to such changes in its
velocity as by the corrosion of its own bed it could effect, then the
formation of such buttresses can be accounted for only by the supposed
permanence of a sheet of water, whether of a temporary lake or of the
arm of the sea, at their levels. Now such projecting masses are
extremely common in the sides of most of the tributary streams of the
valleys. I conclude, therefore, from the consideration both of the beds
of stratified alluvium at the bottom of the main valleys, which there
is the greatest difficulty in believing could have been deposited by
the rivers under the existing conditions; and of the buttresses on the
sides of the hills, which similarly could not have been formed by the
present streamlets, that it is satisfactorily proved that the valleys
of the Spean and Roy have been occupied by a sheet of water which has
slowly and very gradually retired, leaving in almost every part
unequivocal evidence of the check which matter drifted by a current
meets with, when it arrives at or near to the surface of still water.

I have as yet confined my argument to the valley of the
Spean and its tributaries, and to that portion of it which is below the
lower shelf; but I may here add, that it may be inferred from the same
kind of evidence already used, (I allude more particularly to some
buttresses above the 972-feet shelf to the north-east of the houses of
Glen Turet, and to a shelf intermediate between the two upper ones,
Tombhran,) that water long remained in Glen Roy at an altitude above
that which we have as yet been considering, and at other levels besides
those indicated by the three shelves themselves. If, also, we look to
other valleys in this part of the country, we find similar appearances.
For instance, on the flanks of the valley of the Tarf Water, which
flows into Loch Ness (at the elevation of about 1000 feet, near the
bridge, where the road to Garviemore crosses the river), there are
large conical piles, with their summits truncated by a rude terrace,
composed of well-rounded pebbles, sand, and an argillaceous earth in
irregular beds. Some of the layers of sand and fine gravel were in
curves, but slightly inclined; and this structure, together with their

composition, made it at once evident that they must
have been drifted into their present position by currents of water.
Again, near Fort Augustus, the Great Glen, with the exception of the
central part, where the river has worn for itself a broad course, is
filled with irregular strata about thirty feet in thickness of sand,
gravel, and coarse shingle. In the sand some of the layers are most
regularly waved, as if by a tide ripple. These beds are about seventy
feet above the sea; fringes of similar deposits skirt at intervals both
sides of the Great Glen, but where they are present they do not occur,
as far as I was enabled to observe, at a greater height than about 100
feet, that is, than the water-shed of this great valley,—a fact
somewhat analogous to the coincidence in level between the true shelves
or roads and the heads of the valleys in which they occur. At the
south-west end of the Great Glen, nearly opposite to Loch Leven, there
are some extensive flats, which from a distance appear to be similarly
composed, and which in one part have been modeled into two nearly
regular terraces, one rising above the other. A somewhat similar
structure may be observed in a part between Loch Eil and Loch Lochy;
and this structure can only be explained by water having successively
occupied for long periods different levels.

Referring now to more distant points, we find in the
broad valley below Loch Tulla (a tributary of Loch Awe, and the stream
flowing thence enters Loch Etive,) there are some appearances, although
obscure, of the bottom of the valley having been once filled up with
stratified alluvium. On the river Tay, however, near Loch Dochart, the
phenomenon is clearly developed. On the south side there is a long
mound or terrace, about 150 feet high, entirely composed of
well-rounded pebbles mingled in layers with a yellow sandy clay. From
this point to Tyndrum (at an elevation of between 400 and 500 feet
above the sea) there are similar banks of waterworn materials, and in
more than one part I observed a fine white sand, like that on the
seashore. On each side of the valley where it divides, near Tyndrum, a
broad expanse is scattered over with low ridges and flat-topped hills
of equal height, from which it would appear that the whole space had
once been covered up with these deposits. Towards the mouth of the Tay
the terraces and platforms of Strathmore have been remarked by many
observers, on the sides of the small neighbouring valley of the Dighty.
Mr. BLACKADDER,1 in a letter to Mr. LYELL, says, "A narrow track of
gravel, sometimes in the shape of platforms, at others in small
hillocks, very similar in appearance to those of Strathmore, extends to
the height of about 600 feet; and some isolated patches on the southern
face of the Sidlaw Hills occur at a greater elevation." From
expressions used by MACCULLOCH and other writers, I am led to believe
that beds of similar matter irregularly superimposed over each other,
occur on the sides of almost all the valleys of Scotland. In such cases
as in that of Loch Dochart, we have no proofs, as horizontal shelves or
ancient beaches have not been preserved, that the relative level of the
country has remained the same, since the period when it was first
traversed by running streams; and therefore it is not absolutely
certain that the present rivers, with a very different inclination,
might not have deposited the

rudely stratified beds in the lower part of their
courses, and afterwards with an altered velocity have cut through them.
But as we do know that no such change has affected a large neighbouring
region, and as such movements could hardly thus have influenced the
drainage of valleys directed towards different quarters, such doubts
may be overruled. This being the case, the same argument as before used
may be repeated, namely, that the waterworn materials appear to have
been transported by the present rivers, and yet that they are so
deposited as could not have happened without some intervening cause.
The phenomenon demands an explanation; and the only obvious solution is
that which from several and nearly independent considerations was
proved to have been the case with the Spean, namely, that it had been
occupied by an expanse of gradually subsiding water, either of a lake
or of an arm of the sea. This conclusion, therefore, may be urged with
only little less force regarding many, if not all, of the valleys in
this part of Scotland.

It may be asked, of what nature was this sheet of
water? If we suppose a barrier erected across the mouth of each valley,
and a lake to be thus formed, which sunk from the gradual deepening of
its mouth, all the appearances above described would be explained. It
is a startling assumption to close up the mouth of even one valley by
an enormous imaginary barrier; to do this with all would be monstrous.
Of such barriers in the district we are considering I need not say
there does not exist any trace, nor need I repeat what I have already
said against so vain a supposition as that they could have been swept
away by any great debacle from the sides of those hills, of which the
whole alluvial covering has been preserved since the period when the
upper shelves formed beaches, without even a remnant of them being
left; and I may add, that it will hereafter be shown by the clearest
proofs, that the ordinary alluvial action, and likewise that of running
water, even under the most favourable circumstances of a waterfall, has
been far less efficient than could have been anticipated.

But it may be asked, would not the hypothesis of a
succession of lakes explain the appearance, the matter accumulated
above each delta sloping upwards from one level to another. I can only
answer this with respect to those valleys which I have myself seen: in
the Spean, Roy, Tarf Water, and some others, it is easy, as before
stated, to replace in imagination the solid rock; and although some
small lakes* would be

* Sir LAUDER has represented three in
his map (Edinburgh Royal Transactions) by the figures 5, 6, and 7. I
cannot, however, by any means agree with him in the limits thus
assigned to them. Is it meant to be asserted, that there is any barrier
perfect, with the exception of such a gorge as the river is now
cutting, at the lower end of number (7), on a level with the line at
its upper extremity; or so nearly so as to allow of the upper part
being considered as a supralittoral delta? Such did not by any means
appear to me to be the case. Was not the barrier only supposed to have
existed, as in the theory of the shelves? I must also observe that the
fringe or deposit does not terminate a little way within the mouth of
the Roy, as represented by the line marked (7). It appears to me
unfortunate that Sir LAUDER marked the limits of these deposits, which
are accumulated in a gentle slope, in a similar manner as he
has done the shelves, which are horizontal. Any one would
suppose the lines 5, 6, and 7 were horizontal, like those marked 1, 2,
3, and 4, This difference alone indicates a corresponding one in their
origin, as will hereafter be attempted to be shown.

thus formed by the replaced barriers (as probably would
be the case in every valley), the fringe of stratified alluvium we are
now speaking of skirts the valley at an elevation above them. To assume
that these rocky barriers were formerly much higher, and were
demolished by some means independent of the action of the river (for
this action tends only to form a narrow wall-sided gorge, as may be
seen in those barriers which certainly did exist), would be as
gratuitous as the imaginary erection of one great barrier across the
mouth of the valley, and would explain, from the continuity of the
slope, the appearances far less perfectly. Moreover, if the origin of
the sloping fringes could be explained by the assumed former existence
of a chain of lakes, the buttresses high up on the sides of the valleys
clearly could not be so. Nor will any one pretend that any lake-theory
can be applicable to the deposits on the sides of the great valleys,
such as Strathmore, and the Great Glen of Scotland, which terminate in
deep and open friths. Therefore it has not been the water of several
lakes any more than of one lake, which slowly retiring from these
valleys, determined the accumulation of the beds, where we now see
them. There is, then, as we have conclusive evidence that an expanse of
slowly subsiding water did occupy these spaces, but one alternative,
which we are compelled to admit, and this without any consideration of
the shelves themselves, excepting so far as they serve as
artificial levels to show that the country has not been unequally
elevated, namely, that the waters of the sea, in the form of narrow
arms or lochs, such as those now deeply penetrating the western coast,
once entered and gradually retired from these several valleys.

Section IV.—Proofs from organic remains of a
change of level between the land andthe sea in Scotland. The
effects of elevation traced in hypothesis.

Another question immediately arises; did the waters of
the sea slowly subside, or the land slowly rise, the effect in each
case being similar? But first it will be proper to show, from the more
ordinary kind of evidence, that there has been some change of level
between land and water affecting Scotland within recent times, although
not to the amount inferred from the arguments above advanced. Mr. Smith1 of Jordanhill, in an excellent paper*, has lately shown from the
presence of elevated organic remains, that within a period geologically
extremely recent, both the east and west coast of Scotland has been
raised some hundred feet; namely, at Banff and near Glasgow† about 350
feet. Considering the facts given in this paper, it can scarcely be
doubted, without making the most improbable assumptions, that the Great
Glen of Scotland, of which the highest point is only ninety-three feet
above the sea, was within this recent period an open strait; and, I may
add, it must then have strikingly resembled the Beagle Channel in
Tierra del Fuego, an arm of the sea narrower, longer, and straighter,
which intersects the extreme southern part of South America. In ac-

cordance with this fact, I was informed by the person
who now has the charge of the locks on the canal, that when they were
cutting through the gravel at the head of Loch Ness many broken sea
shells were found in the lower part, which appeared to him
like those on the sea-coast. When exposed to the atmosphere they soon
decayed. This point must be between forty and fifty feet above the
level of the sea. There are remnants, as before stated, in this part of
the Great Glen, as well as at the south-west extremity, of coarse
sublittoral formations, which, I suppose scarcely any one would
dispute, were accumulated before that small change of level took place,
which is indicated by the elevated marine remains. That the movement
must have been exceedingly slow, may be inferred from the existence of
so many beaches, each requiring time for its formation, which rise one
above another on both coasts of Scotland. Mr. MALCOLMSON*1 mentions no
less than eleven in Elgin, from the lower one of which he procured
twelve species of existing marine Testacea. On the opposite coast also,
Mr. Smith has described† several ancient beaches between the present
one, and the great terrace, between thirty and forty feet high, which
"forms a marked feature in the scenery of the west of Scotland." It is
also important to observe here, that the supposed greater movement
deduced from the nature of the superficial deposits, is of precisely
the same slow kind, and interrupted (as will presently be shown) by
periods of rest, as this lesser movement, attested by the presence of
sea shells and step-formed beaches. If, then, the Great Glen was for a
long period occupied by an arm of the sea, which very slowly retired
from it, deposits must have accumulated on its shores, and likewise for
some little distance within the mouths of the valleys which entered it.
If we suppose that the sea stood at the same level in the Great Glen as
it lately did both on the east and west coast, then the salt water
would have almost entered Glen Roy, and would have wholly covered that
sloping fringe of gravel, which has been so often mentioned as skirting
the course of the Spean. Whether this be granted or not, after what has
been stated it can hardly be disputed, that within recent geological
periods an arm of the sea entered at least the mouth of the Spean, and
very slowly retreated from it. Remembering that the conclusion was
forced on us by distinct lines of arguments, that a body of water must
have slowly retired from these valleys, and that lakes sufficiently
large to have produced the observed effects could not have existed in
them, may we not, with the additional consideration that some parts of
the deposits here must be of marine origin, deliberately
affirm it proved, that it was the waters of the sea that, even at great
heights, checked and banked up at successive levels, the detritus
brought down by the ancient rivers and streamlets? I am aware that the
argument would have had a greater appearance of strength had
I commenced with the inference deduced from the presence of recent
shells at con-

* Proceedings of the Geological Society,
1838, p. 669. I was informed by an intelligent quarryman that he had
observed many broken sea shells in a gravel-pit, about two miles north
of Grant Town, on the roadside to Forres, and therefore eighteen miles
from the nearest sea-coast.

siderable elevations on both coasts of this kingdom,
but I preferred the method I have followed, because I believe it is
equally legitimate, and of more general application, although at first
not so obvious.

From these facts it is certain that there has been a
change of level affecting within recent times the whole central part of
Scotland, and of a kind very similar to that which has been the subject
of so much attention in Sweden, where, according to Mr. Lyell, remains
of existing marine animals have been raised to the height of between
500 and 600 feet above the sea. The change of level in the case of
Sweden is as certainly known to be due to a slow movement of the land,
and not of the water, as it is on the coast of Chile, where a small
tract is violently upraised during an earthquake, the distant parts of
the same coast being unmoved. It would, however, be quite superfluous
here to enter into this question at length, as it has almost ceased to
be debateable ground*. It may then be concluded that the supposed great
change of level in Scotland, deduced from the foregoing arguments, as
well as that smaller fraction of it attested by marine remains and
ancient sea-beaches, is due to the rising of the land, and not to the
sinking of the waters.

We will now endeavour to trace in hypothesis the
effects which would be produced by an arm of the sea slowly retiring
from inlets during an equably progressive elevation of the
land. In a deserted sound or flat-bottomed valley, surrounded by
mountains, curved lines crossing the river would mark the ancient
beaches. Each of these lines would be higher than its neighbour on the
sea-side, owing to the rising of the land in the interval of their
formation, and would be more distant from the head of the valley,
chiefly on account of the matter brought down by the river, and in some
parts from the natural slope of the fundamental rock. When the upper
line formed a beach, it is evident that the whole of the lower part of
the valley must have been under water, and that the prolongation of the
beach would stretch along the flanks of the adjoining mountains some
way inland from the present shore. In like manner each successive and
lower beach-line would wind along the steep sides of the hills, and
cross the valley further and further from its head. It should be
observed, that although I have spoken of successive beach-lines, yet as
the land is supposed by the hypothesis to rise at a perfectly equal
rate, every part of the valley will have successively formed, during an
equal period, a beach; so that each part having been similarly exposed,
the slope will be uniform; nor will it be possible to distinguish any
one line of beach. Again, if we suppose matter to be removed from the
valley by the action of the tides, instead of being added to it by the
river, yet as an equal quantity (or a quantity insensibly varying from
the varying degree of exposure, as the form of the land slowly changes
during its rise) would be removed at each level, the slope in this case
also would be uniform. In that part of each successive beach, which
winds along the steep flanks of the mountains, it is not probable that
much matter would be added, but the downward descent of some portion of
the detritus, which is

* An excellent summary of the argument
is given by Mr. Lyell in his Elements of Geology, chap. v.1

formed on all land by meteoric agency, would be
checked; but as it would be equally checked at each successive level,
the outline of the mountain would remain unbroken. These same lines,
however, although protected in the more inland parts, might suffer
degradation where exposed to the greater force of the waves near the
mouth of the sound; but the parts differently affected would blend into
each other, and so would it be with each successive beach-line; and the
slope therefore, whether added to or corroded, or left untouched, would
never show the traces of action on any one defined horizontal line. A
little reflection will indeed show that when the water stood at the
highest level, any part or point which happened to be most exposed
would, from the natural slope of all mountains, be some way inland
compared with the same relative point on the present coast; at all
intermediate levels the waves would attack an intermediate part, either
high up and more inland, or lower down and nearer the coast, so that
the line (or rather zone) of greatest littoral action, joining the
parts which were successively most affected, would, under the
conditions of the hypothesis, be inclined with the horizon either more
or less, according to the original inclination of the land. Lastly, the
river in the valley, as it gained power from the sinking of the sea,
would generally remove the central portions, and leave only a fringe of
the littoral and sublittoral deposits. This fringe, although formed by
successive horizontal beach-lines, would slope upwards, as
the whole bottom of the valley would have done if no part had been
removed. I allude to this structure more particularly, because it is
not at first obvious that matter accumulated on a sea-shore would in
any case form a fringe of this kind.

In the hypothesis I have supposed the upward movement
of the earth to have been absolutely uniform during equal periods. But
this probably has seldom been the course of nature. There is clear
evidence that the action of volcanos is intermittent; and the force
which keeps volcanos in action being absolutely the same with that
which elevates continents (as I endeavoured to prove in a paper read
not long since, March 7th, 1838, before the Geological Society),1 so we
must suppose that the elevation of continents is likewise
intermittent,—a conclusion which receives ample confirmation from the
occurrence in nature of successive lines of escarpment, rising one
above another, which mark those periods of rest when the sea wore
deeply into the former coast. Let us then suppose that the water stood
for a longer time at some one level than at any other. The first effect
would be, that the beach or delta at the head of the sound, where the
river is constantly bringing down detritus, would be broader there,
owing to the greater accumulation of matter during this longer period,
than in any other part; and therefore when the bottom of the whole
valley was converted into land, the slope, which is everywhere gentle,
would in that part approach nearer to horizontality; but in other
respects there would be scarcely any difference. In like manner, in
those portions of the mountains, on each side of the valley, where
from the protected nature of the site matter did during the whole rise
accumulate, though very slowly, the line would, from the greater
quantity of matter added during

the longer period of rest, slightly project beyond the
general slope of the surface; and where any rivulet came down a very
little delta would be formed. Also on any projecting or exposed point,
the solid rock would be more deeply cut into than in the other lines.
But as the land rose, the little deltas gently sloping from the line of
ancient beach, with their front part cut off by the action of the
subsiding waters, would project from the hill sides in the form of
obliquely truncated buttresses; to the heads of which the horizontal
lines of beach will exactly coincide, as indeed they likewise will with
the broader ones, where crossing the bottom of the main valleys; but
the slope in the latter case will blend both above and below with the
inclined surface formed by the matter rapidly accumulated at every
successive level. Now it has been shown that Scotland within modern
times has undergone a great elevation; it has been shown to be
extremely improbable that such movements should be equally progressive:
the effects of aqueous action on the surface of the land during the
intermittent periods of rest in the elevatory forces have been traced;
and it will have been perceived by those who have read the early part
of this paper, or the memoirs of Sir LAUDER DICK and Dr. MACCULLOCH,
that the results anticipated in the hypothesis are the characteristic
features, even in detail, of the "parallel roads of Lochaber": I
believe, then, that the hypothetical case gives the true theory of
their origin.

Section V.—Objections to the theory from the
non-extension of the shelves, and the absence of organic remains at
great heights, answered.

Several objections to this view, which implies that the
whole country has been slowly elevated, the movements having been
interrupted by as many periods of rest as there are shelves, will occur
to every one. Perhaps the most important of these is, that, as the
upward movement probably affected a considerable area, or at least as
it cannot be supposed to have been confined within a defined line, so
ought the shelves to be continuous over an equal space. I believe,
however, from what I have seen in South America, that it would be more
proper to consider the preservation of these ancient beaches as the
anomaly, and their obliteration from meteoric agency the ordinary
course of nature. Some contingencies seem absolutely necessary for the
formation of the shelves, such as a sufficient height in the land, a
steep slope, and that the country should be formed of rocks which
afforded an abundance of somewhat adhesive detritus; we may conclude,
moreover, that the surface must have been covered with turf, immediately
after the waters subsided; for otherwise the loose matter would
infallibly have been washed from the hills, and this contingency
implies a protected, and hence, perhaps, an inland situation,
which, at the period, when the water stood at the upper shelves, would
leave but a small area. The abundance of detritus no doubt is quite
necessary; for although the solid rock is in some parts notched, I do
not believe the shelf would anywhere be distinguishable if the soil and
detritus were entirely removed from it. It would also appear to be
necessary that the valley should either have been originally closed at
its upper end,

or that during the period of rest some shallow part in
it should have become so from the accumulation of sediment, or from any
other cause, so that no stream set through it. Thus the two upper
shelves of Glen Roy die away as soon as they enter the valley of the
Spean, which must at the period when the waters stood at their levels,
have formed an open channel connecting opposite seas. That the ancient
beaches in this case extended to that point, beyond which the
accumulation of matter was prevented by too much exposure, seems
clearly indicated, in a manner before explained, by the extremities of
the lower shelf stretching beyond those of the upper. When, however,
the 972-feet shelf existed as a beach, the channel of the Spean was
converted by the closing of the pass of Muckul into a sound; and the
shelf, apparently in consequence, winds along the sides of the valley
both of the Spean and Roy. Besides the requisites here mentioned, the
shelves appear to be more plainly marked where the valley is narrow,
and, perhaps, likewise where it is tortuous. Now from the little I have
seen of Scotland, I very much doubt whether these several contingencies
occur frequently together; they certainly did not in several valleys
which I visited. It must also be borne in mind, that as Sir LAUDER
DICK traced the lower shelf very much further than MACCULLOCH had done,
and as I found a remnant of one in a distinct valley, and especially as
Sir DAVID BREWSTER has seen shelves in two places on the Spey, the
probability is that others, though perhaps obscurely developed, will
yet be discovered. The irregularly shaped area, in which shelves have
already been found, measures in one line twenty British miles, and in
another twenty-five.

Notwithstanding what I have now said, the presence of
the shelves in some of the glens and their absence in others, in the
district of Lochaber itself, is a very extraordinary circumstance. Thus
in Glen Roy three lines are perfectly developed, whilst in the
neighbouring one of Glen Gluoy it appears that only one exists. It is
useless without data to speculate on the nature and force of the tides,
currents, and winds of former periods, or on the kind of vegetation
with which the land was then covered; all circumstances, perhaps,
sufficient to determine the formation or preservation of a mere narrow
mound of soft matter on the steep side of a mountain. But the following
case proves, and it deserves particular attention, that the limits of
the ancient waters cannot even approximately be inferred from the
present extension of the ancient beach-lines. MACCULLOCH has drawn in
his map a shelf intermediate between the two upper ones, on the face of
the mountain (Tombhran) opposite to where Glen Turet joins Glen Roy:
Sir LAUDER DICK has not noticed this shelf*. Perceiving its

* Until I saw this shelf I doubted its
existence, because I had not been able to discover others mentioned by
MACCULLOCH: thus one is figured by him in a ravine branching from Glen
Roy (improperly called by him Glen Fintec), which, though having
ascended it, I was unable to see. Again MACCULLOCH states, that two
shelves occur in Glen Gluoy, whilst Sir LAUDER DICK, who seems to have
examined most carefully this glen, could find only one. I may here
remark, that should two shelves be hereafter discovered there at the
same relative height from each other with those of Glen Roy, and this
is stated to be the case by MACCULLOCH, the fact would be highly
satisfactory on the theory of the shelves having been sea-beaches. From
an excellent point of view, however, on the side of Ben Erin I could
see no trace of a second shelf. MACCULLOCH also

importance I examined it with scrupulous care. It
occurs rather nearer the lower than upper shelf, and as these two are
only eighty-two feet apart, and are here strongly marked, it was
scarcely possible (especially as I purposely looked at it from every
point of view,) to make any mistake in the absolute parallelism of
this intermediate shelf. It can be traced for nearly three quarters of
a mile; at the west end disappearing quite insensibly, like the lines
in Glen Collarig, but at the other end rather more abruptly in a water
course. I walked along its whole length, and its structure is perfectly
characteristic; I refer to the materials of which it is composed, its
breadth and inclination. The two regular shelves are, perhaps, more
plainly marked here than in any other part of the whole glen; and it
would appear probable that this is owing to that portion having been
exposed to a longer space of open water, by which means the ancient
waves acquired a greater than ordinary power in heaping up detritus.
In the mouth, however, of Glen Collarig and of Glen Roy, an exposure to
a wider channel, but at the same time to one open at both ends, and
therefore probably a tide-way, has entirely prevented the accumulation
of matter; and hence the beaches gradually disappear there. This view,
if correct, as I fully believe it to be, shows by what a slight
difference of circumstances, either a remarkable development or an
entire obliteration of the ancient beaches has been determined. The
intermediate shelf clearly owes its existence to the same causes which
have in this part so strongly marked the upper and lower one; and
though it is less strongly marked than these two in this immediate
neighbourhood, yet it differs but little from them as they ordinarily
occur, and is, I think, fully as plain as the lower shelf throughout
Glen Spean, I assert, then, that it is an incontestable fact, that
water must have remained at the level of this intermediate shelf for a
long period, and only a little less long than at the other
lines; yet in no other part of Glen Roy, the valley where circumstances
have been so pre-eminently favourable for the formation and
preservation of these beaches, a trace of this intermediate shelf has
been observed. It has likewise been most clearly shown, that barriers
could not have existed at the double mouth of Glen Roy, and we have
seen that the surface of the land has been preserved in that
neighbourhood in a manner quite extraordinary; yet it is known on the
authority of Sir LAUDER DICK, who appears to have examined the whole
course of the Spean and its tributaries with great care, that not a
vestige of either of these upper shelves can be discovered beyond the
mouths of Glen Roy. Any argument, therefore, whatever, from the
non-existence of the shelves or beaches bearing on the former limits of
the ocean over this part of Scotland, during the period of rest in the
subterranean movements, is valueless.

figures a supernumerary shelf at a point
north-west of the houses of Glen Turet, at a level above that of the
upper shelf of Glen Roy; a mound of alluvium, above, and nearly
parallel to the shelf, certainly occurs there; but from the want of
sharpness of outline, I should be unwilling to pronounce that it had
formed a line of beach, although I should be far from feeling any
surprise if this could be shown to have been the case.

In the valleys of the Spean and the Roy, I attentively
examined, with the expectation of finding fragments of sea shells, the
matter accumulated on the shelves, and more especially the thicker beds
of gravel and sand which occur at lower levels; but I could not
discover a particle, and the quarrymen assured me they had never
observed any. This may at first be thought a strong objection against
the theory of the marine origin of these deposits. But having been led
in consequence of Mr. MURCHISON'S1 remarkable discovery of recent sea
shells in the inland counties of Shropshire and Staffordshire, to
examine many gravel pits there, and having observed how frequently it
happens, that not the smallest particle can be discovered in vast
accumulations of the rudely stratified matter, and that when found, the
fragments are generally exceedingly few in number and partially
decayed, I feel convinced that their preservation may be considered as
a remarkable and not as an ordinary circumstance. After a longer
interval of time, or under some slightly less favourable conditions,
all the gravel beds of Shropshire, which no one can doubt were
accumulated beneath the sea, would be as destitute of organic remains
as those of Lochaber. In some parts of South America I have found beds
of gravel which did not contain a fragment of shell, and yet on the
bare surface, nearly perfect ones were strewed in numbers. Mr. SMITH
describes* beds on the west coast of Scotland, and Mr. LYELL† others in
Sweden, undoubtedly of marine origin, but wholly destitute of organic
remains. On the coast of Forfarshire also Mr. LYELL, as I am informed
by him, found shells in gravel beds extending to the height of between
fifty and sixty feet; but at greater altitudes similar beds occur which
do not contain any: he has observed the same kind of fact strikingly
illustrated in Norway‡. It is easy to imagine several

* Edinburgh New Philosophical Journal,
vol. xxv. p. 380.

† Transactions of the Royal Society,
1836, p. 11. and 15.

‡ Mr. LYELL has had the kindness to
give me the following observations on this point.

"In the country surrounding the fiord
of Christiania, especially between Christiania and Dramman, and between
Dramman and Holmstrand in Norway, deposits of clay and sand rest in
horizontal beds on the gneiss, granite, porphyry, and other rocks.
Large masses of this sand and clay reach in some places to elevations
of more than 600 feet above the level of the sea, and nearly fill many
upland valleys; but it is only in those patches which occur at the
height of about 200 feet, and usually less than fifty feet above the
sea, that shells (all of recent species) have been found. This sand and
clay appear to have accumulated on the older rocks during their gradual
upheaval from beneath the sea, so that greater elevation becomes a test
of higher antiquity, and those patches which are found at small heights
near the borders of the present fiord are very modern. Even in these
last the shells are often in so advanced a state of decomposition as
greatly to favour the theory that a more considerable lapse of time
might be sufficient to obliterate all traces of their existence. Thus
for example, on the banks of a small river about two miles above
Töusberg at the place where the bridge crosses it, a section of loamy
clay is laid open, the lowest part of which cannot be raised more than
a few feet above the salt water of the fiord of Christiania. In the
upper part of the mass for a thickness of fifteen feet no fossils can
be detected, but somewhat lower faint casts of the Mytilus edulis,
chiefly indicated by purple stains, are observable. Still lower down
more perfect specimens of the same shell, together with Cardium
edule, occur, but both in so soft a state as to crumble into dust
when dried. With these the more solid Cyprina islandica and Saxicava
rugosa are occasionally found, and although soft when first taken
from the matrix are capable when dried of being preserved entire. If in
the short period which has probably passed away since these shells

1 Roderick Impey Murchison (1792-1871), army officer and one of the foremost geologists of the era, president of the Geological Society of London, 1831-1833 and 1841-1843, president of the BAAS, 1846, president of the Royal Geographical Society of London, 1843-1858 and Director-general of the Geological Survey of Great Britain, 1855.

circumstances which might determine the preservation or
decay of the shells; even on the assumption, which is not necessary,
that they have in all such cases been imbedded. Thus in Shropshire, the
gravel is covered in most parts by an earthy deposit, which contains a
small proportion of lime; hence the rain water having absorbed carbonic
acid gas in its descent, would find matter to dissolve before it
reached the layers containing shells; whereas in Lochaber the gravel
and sand, being derived entirely from granite rocks, does not, as I
ascertained, usually contain any free carbonate of lime, and
consequently the fragments of shells would more readily be dissolved. I
do not wish to assign this circumstance* as the real cause of their
disappearance, but merely to indicate it, and other similar ones, as
quite sufficient to show that the marine origin of the shelves cannot
be controverted from the absence of organic remains.

Section VI.—Application of the theory to some
less important points of structure in thedistrict of
Lochaber, and recapitulation.

By considering the hypothetical case above given, I
think it was shown that the proposed theory explains every essential
point in the phenomenon of the parallel roads. And I will now endeavour
to show how far it applies to some minor points of detail. For
instance, I have described a horizontal band of rock on one side of the
narrow mouth of Loch Treig, with its face worn into smooth concave
forms, like those over which a water-fall rushes; and on the other
side, a great spit or bank of sand and gravel. Now on the belief, that
a sheet of water seven or eight miles long, and two or three broad, was
drained during each ebb-tide to the depth of several feet through a
narrow curved channel, and then again raised by the following tide to
its former level, the effects there produced are quite intelligible. It
is also easy to perceive, that through the means of the tidal action,
points of solid rock might have been obliquely cut off in the same
manner as on existing beaches; and that flat channels, resembling in
every respect those which at present frequently separate small

near Töusberg were imbedded, the
progress of decay can have proceeded so far, we may well suppose the
percolation of water during antecedent ages of indefinite extent to
have destroyed all signs of fossils in the more ancient and elevated
patches of loam found more than 500 feet high in the adjacent hilly
country."

* I may observe that it very frequently
happens, that shells are found only at some depths in these superficial
deposits: this is the case in several of the gravel pits in Shropshire;
in cutting the canal at the head of Loch Ness, the shells were met with
at the bottom, whereas, the layers nearer the surface, as I can vouch,
contain none. Mr. SMITH speaking (p. 380 and 391. vol. xxv.
Philosophical Journal,) of the clay beds on the west coast of Scotland,
says, that the marine remains with which it abounds "are almost
invariably found in the lower part of the bed." I infer that in all
these cases shells originally existed in the upper part, but have since
decayed: Mr. SMITH, however, offers a different explanation. In the
extensive and superficial beds of elevated shells on the coast of Peru,
where rain does not fall, and where consequently loose matter is not
washed from the surface, I have traced as I have ascended from the
beach a most perfect gradation in the decay of the shells, until a mere
layer of calcareous powder, without a vestige of structure, alone
remained.

islands from larger ones, might have been worn between
hummocks (such as those on one side of Meal-derry) and the lines of
shelf.

If, again, we consider what must take place during the
gradual rise of a group of islands, we shall have the currents
endeavouring to cut down and deepen some shallow parts in the channels,
as they are successively brought near the surface, but tending from the
opposition of tides to choke up others with littoral deposits. During a
long interval of rest in the upward movements, from the length of time
allowed to the above processes, which essentially require time (though
they are favoured by the rise of the land rather than by its
remaining stationary), the tendency would often prove effective both in
forming by accumulation of matter, isthmuses, and in keeping open
channels. Hence such isthmuses and channels just kept open, would
oftener be formed at the level, which the waters held during the
interval of rest, than at any one other. These isthmuses and channels
when left by the receding waves, might be called land-straits, for they
would present smooth, flat, narrow surfaces, connecting more open
spaces. During the rise of the land they would at first separate the
heads of two adjoining creeks, and afterwards, the upward movements
proceeding, they would form the watersheds between adjoining and
opposite glens. By this means, I explain both the ordinary structure of
the land in these mountains, where the waters divide, as already
described; and more especially the remarkable fact of the exact
coincidence of several such points with the lines of shelves,—the
shelves only indicating the long interval of rest in the upward
subterranean movements. It may be remembered that I described at the
head of the Roy and of the glen near Kilfinnin, patches of alluvium or
remnants of terraces on the sides of the land-straits, a little above
the flat where the waters divide. This structure is in perfect
accordance with the theory that drift matter began to accumulate in
such parts at that period, when the tides in them were first checked,
or otherwise affected by the rising of the land; and that the channels
were finally closed at their present levels, solely from the long
interval during which the sea acted at such levels. Hence, also, we
might have expected, that patches of alluvium would occur (as is the
case) on the sides both of the land-straits which are, and those which
are not connected with shelves at corresponding levels.

From the levels taken by Mr. MACLEAN1 with Sir LAUDER
DICK, it appears that the upper limit of the Glen Gluoy shelf, which
coincides with the division of the waters, is twelve feet higher than
that of Glen Roy. The intervening space is nearly a mile in length,
moderately broad, and very flat, having only a fall of the twelve feet;
and Sir LAUDER states* that he saw in this part the surface of the
solid rock in the bed of the little stream. These facts seem at first
to indicate that two periods of rest had supervened, one when the
water stood at the level of the Glen Gluoy shelf, a second when at the
upper level of Glen Roy after a rise of twelve feet, and that,
nevertheless, the effects of these two periods of rest were confined
respectively to separate, though closely adjoining glens. This
circumstance if so interpreted, although improbable in

the highest degree, could not be considered as
subversive of the theory, after it has been ascertained that the upper
shelves of Glen Roy are not prolonged into the valley of the Spean, and
that the short intermediate one in Glen Roy does not extend for more
than three quarters of a mile in that valley. There is, however, I
suspect a more satisfactory explanation. In the First Narrow of the
Strait of Magellan, the tide rises about forty feet, as Captain FITZROY
informs me, whilst eighteen miles to the west at Gregory Bay, the rise
is only about twenty feet. Here then, and other instances might be
adduced, in a distance of eighteen miles, the surface of the water must
slope no less than twenty feet. Let us suppose a rocky barrier
(and that of Glen Gluoy is rocky) to be elevated, by such movements as
those now in progress in South America, across the strait, separating
it into two portions. Might we not expect that the high water mark
would rise several feet higher, in that portion of the former channel
which was still open to the sea subject to the great tidal movement,
than it would in the other connected only by tortuous passages with a
different sea, where the rise of the tide was small? In such a
labyrinth of channels as this part of Scotland must have presented when
the sea stood at the level of the upper shelves, it is even probable
that there would be inequalities in the rise of the tide in different
parts; I conclude therefore that when the rocky barrier was upraised
between Glen Gluoy and Glen Roy, a greater tide-wave, proceeding direct
from the line of the Caledonian Canal, then a great strait, swept up
this deep creek; whereas a smaller one reached by a circuitous course
the Bay of Glen Roy, which, moreover, was connected by some
other straits with the eastern sea.

Whoever walks over these mountains, and believes that
each part has been successively occupied by the subsiding waters of the
sea, will understand many trifling appearances, which otherwise, I
believe, are unintelligible. Thus in Upper Glen Roy he will see in the
level expanse, an old bay, filled up and leveled with tidal mud. Again
at the Gap of Glen Collarig, with its flat bottom and cut off sides
like
a gateway, he will recognise a channel, at last choked up with matter
drifted by the tides, and now left in the state in which it was when
the waters retired from it. The traces of supernumerary shelves will
offer no perplexity to him, and will equally receive with the others a
simple explanation. By the theory of the sea having acted at successive
levels over the whole surface of the land, the great beds of shingle*
and sand,

* I have before alluded to the fewness
of the well-rounded pebbles near the upper shelves, excepting at the
heads of the valleys, or on flat places. This is a difficulty; though
it is one common to many regions, where we know that much denudation
has taken place at some period or at another. Pebbles of most rocks may
in the course of time decay, but those of quartz I should think
(although SCORESBY1 says this rock yields to the frosts of Spitzbergen)
would be imperishable: if so, how comes it that quartz pebbles are not
scattered over the surface of every mountain in which that rock is
present, and in which the form of the land, its denuded state, or the
presence of truncated dikes show that it must once, although perhaps
countless ages since, have been beaten by the waves of the sea? Such
pebbles, however, are not found on every mountain thus circumstanced:
the explanation, I presume, rests in this; that every cause of
disturbance, wind, rain, earthquakes and the fall of fragments all tend
to move the pebbles in one direction alone, namely downwards. I am
inclined to believe this view is

such as those near the mouth of the Spean, have a
cause assigned to them adequate to the effect. Lastly, the manner in
which the deposits near the mouths of the larger valleys have been
modeled into successive terraces, which in some parts at least appear
not to have been formed by the river, receives elucidation. I may add,
that in South America I have observed numerous instances of terraces in
every respect similar to these, with sea shells abundantly scattered on
their surface; and therefore where there could exist no obscurity
regarding their origin.

In concluding this part of my paper I will recapitulate
the course of the argument pursued. 1st. It is admitted by every one
that the horizontal shelves are ancient beaches. 2nd. I showed that no
lake theory could be admitted on account of the overwhelming
difficulties in imagining the construction and removal at successive
periods of several barriers of immense size, whether
placed at the mouths of the separate glens, or at more distant points.
3rd. The alternative that the beaches, if not formed by lakes, must of
necessity have been so by channels of the sea, was not advanced, only
because it was thought more satisfactory to prove from independent
phenomena, that a sheet of water gradually subsiding from
the height of the upper shelves to the present level of the sea,
occupied for long periods not only the glens of Lochaber, but the
greater number, if not all the valleys of this part of Scotland; and
that this water must have been the water of the sea. 4th. It was stated
(the strongest argument being the ascertained fact of the land rising
at the same time in one part and sinking in another,) that in all cases
the land is the chief fluctuating element; and, therefore, that the
above change of level in Scotland, independently attested by marine
remains at considerable heights on both the eastern and western coasts,
implies the elevation of the land, and not the subsidence of the
surrounding waters. 5th. It was shown that in all such prolonged upward
movements it might be predicted, that there would be intervals of rest
in the action of the subterranean impulses. 6th. By an hypothetical
case, the land was subjected to the above conditions, and its surface
was found to be modeled in a manner wholly similar, even in detail, to
the structure of the valleys of Lochaber as they now exist. 7th. The
true theory being considered thus established, objections to it from
the non-extension of the shelves, and from the absence of organic
remains at great altitudes, were answered and shown not to be valid.
8th. Many points of detail in the structure of the glens of Lochaber,
were shown to be easily explicable on the supposition, that the valleys
had been occupied by arms of a sea subject to tides, and which had
gradually subsided during the rising of the land. Having attentively
considered these several and

correct, and that in the course of time,
such pebbles are all rolled down, from having found on an isolated
mountain of quartz in South America (the Sierra Ventana) a superficial
patch of conglomerate, like part of an old beach, which seemed solely
to owe its preservation to the pebbles having been cemented to the
parent rock by oxide of iron, in the same manner as not unfrequently
may be observed on some existing sea beaches. In the case of the
shelves of Lochaber, it is probable, that only a few pebbles were
originally formed, owing to the small power of the waves on the steep
and protected shores of these ancient sounds.

independent steps of the argument, the theory of the
marine origin of the "parallel roads of Lochaber" appears to me
demonstrated.

I may here remark, that MACCULLOCH seems to have been
aware of the great difficulties attending his theory: but having proved
that the roads could not be works of art, or the effects of any great
debacle, he argued, to use his expression from the dilemma of the case,
that they must have been formed on the shores of a lake. The idea of a
continent slowly emerging from beneath the sea, appears, and it is a
very curious point in the history of geology, never to have occurred to
him as a possibility, although he was so bold and ingenious a
speculator. His paper was read in the beginning of 1817, and when we
reflect that during the few latter years, proofs of such movements have
accumulated from all quarters of the world, we must recognise how much
of this all important change (the foundation-stone, I may add, of this
paper) is due to the Principles of Geology by Mr. LYELL.1

Section VII.—On the erratic boulders of Lochaber.

I will now pass on to some other considerations which
partly derive their interest as dependent on the truth of the foregoing
theory. I have said, that the parent rock of many of the fragments
lying on the shelves is not found in the immediate neighbourhood. These
erratic boulders are generally of granite, and are from one to five and
six feet in diameter; they are not confined to the shelves, but are
scattered on the sides of the mountains. On the summit of the insulated
hill of Meal-derry, above the level of the 972-feet shelf, there was
one of large size, together with some well-rounded pebbles of rocks,
which, I believe, do not occur there. In the gap of Glen Collarig the
boulders on and near the upper shelves are frequent, as they likewise
are in the pass between Upper and Lower Glen Roy; they occur also
abundantly at the bottom of the latter valley, and on the side of
Tombhran. From having found them in almost every part which I examined,
I have little doubt that they are distributed in numbers over all the
valleys and mountains, at least, to an elevation as great as that of
the upper shelves: I make this latter restriction, because having
ascended the mountains only in a few places above that level, I cannot
speak positively with respect to the greater heights. On the mountains,
however, between Glen Roy and Glen Gluoy on a hillock north-north-west (magnetic)
of the summit of Ben Erin, I found several masses of granite, one of
which was four feet by three in width and two in thickness (together
with a couple of pebbles from rocks not in situ) resting on
the surface of the gneiss. This hillock seemed to be entirely composed
of the latter rock, and it was separated from all other hills by a
valley. On the flanks of Ben Erin at about the same level, there were
several boulders of granite, one of which was six feet across. Of those
on the hillock (probably there were many others which I did not see in
merely crossing the mountain,) the highest one was found by comparison
with the Glen Gluoy shelf (by means of the barometer), to be 2200 feet
above the level of the sea. I will describe in detail the spot where I
found one other boulder,

in as much as the whole of the district being composed
of gneiss, it might be suspected that patches of granite occurred high
up on the slopes of the mountains, and that the fragments had simply
rolled down into their present positions. This, however, could not
have happened in the case last described, nor in the following one:
about twenty feet below the summit of a very sharp peak (1600 to 1700
above the sea) the whole of which consisted of tortuous layers of
gneiss, there was a block of syenite with pink felspar, two feet eight
inches across. The peak is wholly separated (as shown in the wood-cut,
fig. 4.) from a lofty mountain also of gneiss, by a broad and
quite flat valley, the highest part of which is 215 below the spot
where the boulder lay. I may observe that I did not anywhere see
another boulder of the syenite, nor a single one of granite on this
side of the mountains, which is separated by a lofty ridge from the
valleys of Glen Roy and Glen Gluoy, where the blocks of granite are so
numerous. Between two branches, however, of the Tarf Water (which
enters Loch Ness near Fort Augustus) on the summit of a hillock of
gneiss, about 1200 feet above the sea, I noticed one of granite.

Fig. 4.

A. Lofty mountain of gneiss.

B. A peat moss 215 feet below the
boulder, dividing the waters flowing on each side round the hill C.

C. Boulder of syenite resting on gneiss
1600 or 1700 above the sea.

D. Habercalder in the great glen of
Scotland.

The granite of all the boulders which I observed in
Glen Roy, and likewise of those on Ben Erin, has a uniform character;
it is subject to much disintegration, and therefore I do not doubt that
the boulders were originally much larger. In MACCULLOCH'S Geological
Map of Scotland, the nearest granite in situ to the boulders
on Ben Erin is seen to be at the source of the Roy, near Loch Spey, a
distance in a north-east line, passing over mountain and
valley, of between five and six miles. The granite there has the same
lithological character with that of the boulders, and I do not doubt
that it is the parent rock, at least, of those strewed along the course
of the Roy. With respect to the boulders on Ben Erin, they are
completely cut off from every granitic district by valleys, the highest
point of which is 920 feet below that boulder, the altitude of
which I measured; that is, it would be impossible to walk from granite insitu to these boulders without ascending at least that number
of feet.

I will only further add, that if a sheet of water were
raised to the level of the Ben Erin boulders, there would be a line of
open communication* between them and the granite of Loch Spey; although
I must confess I much doubt whether in that case any of the rock in
situ at Loch Spey would remain uncovered; and if so the origin of
the boulders must be more remote. The other tracts, where granite is
represented in MACCULLOCH'S map, are more distant, and are separated by
deeper and broader valleys from the points in question. From my limited
examination of the district of Lochaber I am unwilling to generalize
respecting the position of the boulders, but I think that they
certainly occur most frequently on the summits of little peaks, such as
on Meal-derry, or on that one of which a wood-cut has been given; and
perhaps likewise in the narrowest parts of the valleys; for instance at
the junction between Upper and Lower Glen Roy. I observed also a
greater number on the shelves than I should have anticipated, from some
of those, which had originally stood higher, having rolled down. But, I
repeat, I will not positively say that such is the case; although with
respect to the boulders on the peaks, as I observed five well-marked
cases, even during my short examination of the country, I have little
or no doubt that the observation is correct.

On any conceivable theory of the transportation of
erratic blocks, whether by some overwhelming debacle, or by floating
ice, or any other means, it will at once be evident that they must have
been scattered over the country, either before the shelves were formed,
or at the time of their formation, but not on account of the delicacy
of the lines at any after period. According to the generally received
opinion of geologists, the so-called "erratic block period" is recent,
and therefore we obtain a rude method of estimating the age of the
shelves, and consequently of the elevation of the whole central part of
Scotland, at least to a height of 1278 feet (or that of the upper
shelf) above the sea.

It may perhaps be worth while briefly to compare
together, under the conditions here afforded, the two theories of the
transportation of erratic boulders, which are alone worthy of
consideration, namely, that of great debacles and of floating ice. I
will not lay any stress on the difficulty of imagining, in accordance
with the first theory, a rush of water so impetuous as to transport
vast masses of rock across profound valleys and up the steep sides of
high mountains, for this difficulty has no special reference to the
case of Lochaber; but those who believe in the past occurrence of so
terrific an agitation of the waters of a deep sea, must in some manner
account for the frequency of boulders in the most exposed places on the
summits of hillocks, and likewise for so many having been left in
narrow straits, where one would have anticipated the most impetuous
rush of water. On the face of Tombhran I observed many boulders
scattered on the shelves, which have been formed there not

* This is a similar fact to what has
been observed on the Jura. Sir JAMES HALL1 (Edinburgh Royal
Transactions, vol. vii. p. 143.) says "it is principally where the
snowy summits are visible from the face of the Jura by means of some
depression in the intervening hills, that we find these travelled
masses."

1 Sir James Hall (1761–1832), geologist and chemist and advocate of the geological theories of James Hutton.

only by the accumulation of loose matter, but also by
the deep excision of the solid underlying rock. Again, there were other
boulders on the shelves on the rocky peninsula near the junction of
Upper and Lower Glen Roy, where much of the gneiss has been worn away.
Here it was not possible, from the non-existence of higher land, that
the boulders could have rolled into their present places from above,
after the formation of the shelves; nor was this at all probable in
several parts of Tombhran. On the supposition of the boulders having
been originally scattered over the country, and the shelves formed at a
subsequent period, we have the difficulty, though perhaps not an
insuperable one, as we do not know their original size, of believing
that blocks of granite have been preserved for a long period on those
very places, where a zone of gneiss had been cut into and worn away.
Some of the boulders on Tombhran were lying on the surface of the lower
edge of the shelves, in parts where, as above said, I fully believe the
inclination of the ground was so trifling that it was impossible they
could have rolled down from above; but I regret much that I omitted,
from not having perceived its importance, to ascertain this point with
certainty. If the fact be so, and I scarcely doubt it, it would prove
that some action, so quiet as not to have disturbed the small quantity
of earth and little stones, of which the shelves are formed,
transported these boulders across deep arms of the sea, and left them
on the surface of the ancient beaches. The theory, that all erratic
blocks, circumstanced like these of Lochaber, have been transported by
floating ice, wholly removes these difficulties; for the icebergs, in
the first place, would generally land the fragments, with which they
were charged, on the lower part of the beaches or shelves; and
secondly, those which had arrived not long before a fresh elevation
would have been exposed only to a small amount of tidal degradation.
Thirdly, the icebergs would frequently be stranded on shoals and
islets, over and round which the tides swept; and likewise they would
be frequently driven on shore in the narrow parts of the channels,
where the waters were pent up. So that in after times, when the land
was drained, it is easy to perceive that the boulders would lie
scattered in such places, as they now actually occupy in the district
of Lochaber. Lastly, this theory requires that every district where
boulders are found should have been covered by the sea; here we have
independent proofs that such was the case, at least to an elevation of
1278 feet.

In my Journal during the voyage of the Beagle, I have
endeavoured to show that the erratic blocks of central Europe were
probably transported at that period*, when

* I refer, of course, only to the more
temperate and central parts of Europe, but it appears that boulders are
sometimes transported in these regions, even at the present time. Sir
JAMES HALL, in his Memoir on the "Revolutions which have affected the
surface of the earth" (Edinburgh Transactions of the Royal Society,
vol. vii. p. 157.),1 states that in the Solway Firth (and therefore in
salt water) "a large block of stone, four or five feet in diameter,
lying within high-water mark, and well known as having served as the
boundary of two estates, was during a stormy night in winter
transported ninety yards, and the persons on the spot were convinced
that this migration was performed by means of a large cake of ice;
formed round the stone, and attached to it, and that the whole had been
lifted and carried forward by the rising tide. The course of this stone
was

its climate was more equable (chiefly consequent on the
larger area of water), which favours a low limit of the snow line, and
therefore the probability of glaciers, the parents of icebergs,
descending in favourable places into the sea. It is therefore to this
period, if this view be correct, that we must refer the "parallel roads
of Lochaber," and consequently the elevation of the land, not only of
the 1278-feet portion (which it is certain has been elevated at an
epoch not distant), but likewise of the whole altitude, whatever it may
be, at which boulders occur. If there be others, as is most probable,
at a greater height than that one on Ben Erin, which I observed in
merely crossing the mountains at a point 2200 feet above the sea, then
by so much the greater has the elevation of the land been within this
same period. Mr. BLACKADDER (in a letter to Mr. LYELL) states he has
seen on the west coast of Scotland, in the island of Mull, large
fragments of quartz rock at the height of 2000 feet, of the same
description as that found on some of the adjoining islands and
mainland. In Sweden M. SEFSTRÖM1 says that boulders occur at an
elevation of 1500 feet; in Massachussets, in North America, they are
found, according to Professor HITCHCOCK,2 at 3000; and on the Jura it is
well known they occur, from low down, to an altitude of 4000 feet. It
is interesting to discover, that in our own country the upward
movements, within the same period, have been more than half
as great as those which have affected the latter colossal chain. But
regarding the exact period, allowance must be made, since on the one
hand the glaciers of the Alps, situated ten degrees nearer the equator
than those on the mountains of Lochaber, must have much earlier
retreated upwards, and failed in descending to the level of the sea,
during the change from the former to the present climate; whilst, on
the other hand, to counteract the equatorial influence, they were
appendages on a greater mass of snow accumulated on far loftier chains.

Section VIII.—On the small amount of alluvial
action since the formation of the shelves.

I now pass on to another consideration. MACCULLOCH was
much struck with the fact, that in many cases where a shelf crossed a
rivulet, I mean one of those silver-like threads of water which descend
the flanks of steep mountains in nearly straight

marked upon the sand below by a deep and broad
furrow,
which remained visible for a long time afterwards, as I have been
informed by several members of the Society, who saw it after an
interval of more than a year." I presume from the position of the stone
as a land-mark, and from the distance it was transported by the rising
tide, that the furrow left by its passage must have been either
oblique or parallel to the shore. What would have been the effect if
this large and heavy block had been pushed over a surface of solid rock
instead of sand? This question will recall to the mind of those who
have read the late papers of Messrs. CHARPENTIER, VENETZ, and AGASSIZ,3 the case of the longitudinally and obliquely scratched rocks of the
Alps. In the Addenda to my Journal during the voyage of the Beagle, I
have endeavoured to show that the passage of ice, with imbedded
fragments of rock, acting at successive levels on the surface of shoals
during the gradual rising of the land, offers the most probable
explanation of the scratches and grooves, which have justly excited so
much attention in Scotland and other places.

3 Jean de Charpentier (1786-1855), German-Swiss geologist who studied Swiss glaciers.
Ignaz Venetz (1788-1859), Swiss engineer, naturalist, and glaciologist. He was one of the first to propose glaciers as a major force in shaping the earth.
Jean Louis Rodolphe Agassiz (1807-1873), Swiss-born American zoologist, glaciologist, and geologist. He would later become a bitter opponent of Darwin's evolutionary views.

lines, it frequently entered a little way on each side
of the gully. From this fact it is evident that the gully must have
been partly formed before or at the time when the shelves were
sea-beaches. I particularly observed several instances of this
structure. One which struck me most was in Glen Roy, opposite a gap in
the mountain which leads to Glen Fintec; here two small threads of
water were united at the point where the line of shelf crossed them,
and at their junction the rock was much exposed, so that any one would
have supposed that the furrow in which they flowed had been entirely
hollowed out by their action. But the shelf curved in a little way on
each side; and, what was more curious, the apex of turf above the point
of junction of the two streamlets had evidently originally formed part
of the shelf. By this it was shown that the entire hollow, with the
exception of the actual beds of the streams, must have existed as an
indentation or little cove on the line of ancient sea-beach. It
appeared to me that the extent to which the shelves entered these
furrows did not bear any close relation to the power of the streamlets
now flowing in them: thus on Tombhran (in front of the houses of Roy) a
great gorge which is impassable, and where the rock is bare and
shattered, has been deeply cut into by the winter torrents, and yet the
shelves enter only a very little way on each side; whereas in other
cases we find a hollow or creek of some size, but with an insignificant
stream flowing in it, for instance, that opposite the gap of Glen
Fintec, which has not even removed the remnants of the shelf from the
head of the gully, in which it has flowed ever since the retreat of the
sea.

Without entering here into a full consideration how
these gullies were originally formed, and whether the indentations made
in the beach at one level might not be produced downwards to another, I
will only remark, that the sea in most situations certainly does alter
the form of its coast, and yet that an accurate map of any shore gives
a line indented in such manner, that a series of them, if placed one
above and a little behind another, would produce the same kind of
furrowed surface which characterizes the mountains of Lochaber, as well
as most others. I will further observe, that when travelling along the
shores of northern Chile and Peru, where the alluvial action is reduced
to an exceedingly small measure, and where it is not probable that
within a recent period there has been any great change of
climate, I was repeatedly much surprised at observing how absolutely
similar all the minor inequalities of the surface (yet covered with
beds of sea shells of existing species) were to those of countries,
where almost every detail in outline is usually attributed to meteoric
agency; I could perceive only one difference, namely, that the larger
valleys had unusually flat bottoms. Although fully convinced of the
truth of this fact, I confess I was astonished at discovering in the
mountains of Scotland, which have been exposed during a vast period to
the destroying action of a wet and boisterous climate, clear proofs
that almost every furrow and inequality has been left nearly in the
state in which we now see it*, by the retiring waves of the sea. From
the preservation of

* It is scarcely possible to convey by language any
accurate idea of the kind of inequalities which, from the

some of these beaches, one can point to the very spot,
and declare so much was removed when the sea stood there, and so much
since by the running streams of fresh water.

It may be asked, has the present alluvial action done
nothing here? Something it assuredly has done, but I repeat,
comparatively nothing to that which was effected before the sea
retreated. In Chile I concluded that the action of the more rapid
rivers and torrents was chiefly confined to removing the littoral and
sublittoral deposits left by the arms of the sea; and secondarily in
cutting, as soon as the upper beds were removed, a wall-sided gorge
through the solid rock. It appeared, that as long as the river had its
passage through the water-worn materials, from the great facility with
which it changed its course, its bed was broad, but as soon as it
reached the solid strata it became exceedingly narrow. These
conclusions are in strict conformity with what I observed in the glens
of Lochaber. Of the small amount of corrosion effected since the sea
stood at a level of the upper shelves, there are some curious
instances. Sir LAUDER DICK, in describing in detail the head of Glen
Gluoy*, concludes that the river has worn there, during the immense
period which must have elapsed since the water (of the sea) retired
from the 1278-feet shelf, a remarkable chasm, between fifty and sixty
feet in depth, but only a few feet wide. The stream in the
northern arm of Glen Turet has cut for itself a passage in the solid
rock in only a part of the valley, between the middle and the 972-feet
shelf. In Upper Glen Roy the southern stream falls into the plain by a
cascade, to the upper edge of which on each side the 1226 shelf
approaches close. I did not ascend the spot, but as far as I could
judge, the water has not cut back more than at most a few yards, into
the rock over which it falls. Other similar instances might be adduced.
Although none of these streams form great bodies of water, yet when
flooded by the winter rains they cannot be inconsiderable; and their
action has been prolonged for so vast a period, that the geographical
features, together probably with the climate of the country,

shelves passing over them and into the intervening
hollows, we know were so left by the sea. I hope any one who feels
interested on this subject, will carefully examine the plates
accompanying Sir LAUDER DICK'S paper (Edinburgh Transactions, vol.
ix.), and especially Plate IV. The shelves on the left side (looking up
the glen) bend into all the principal gullies; and on the right side,
directly in front of the foreground, by looking close at the plate they
will be seen to curve a little way into each of the perpendicular
furrows (some thinning out and others commencing), the bottoms of which
have evidently been much deepened by the descending streamlets. The
idea given by these plates of the state of surface in these mountains,
and of the manner in which the shelves bend round the headlands and
enter the gullies, appears to me exceedingly faithful; although the
glen itself, as represented, is too narrow and profound, and the sides
much too steep. To view this Plate is a lesson full of instruction to
the geologist, for he will scarcely fail to be astonished when he sees
that the drawing is characteristic of any ordinary valley in a
mountainous country, and at the same time to find himself compelled to
admit, that even the little furrows, which it might be thought had been
formed but yesterday, must have owed their origin, at least in great
part, to the successive coves or indentations, continued one below
another on ancient sea-beaches.

have been greatly changed. The rocky crests of the
mountains no doubt have suffered from the weather; but the perfection
of the shelves over spaces many hundred yards in length, and in the
case of Glen Roy (where the three shelves occur) of some hundred feet
in vertical height, clearly proves that as the sea left the greater
part of the surface, so does it now remain. Amongst mountains the
bursting of temporary lakes may sweep away or accumulate vast
quantities of rubbish in the valleys; earthquakes may hurl down piles
of fragments; and torrents during the lapse of ages, or under
favourable conditions (such as the descent of many pebbles), may
excavate a gorge of almost any depth, but which, as far as it is
possible to judge, will always be narrow and steep-sided. All this must
often have happened, and will so again; but the glens of Lochaber
plainly show that the effects of ordinary alluvial action is
exceedingly small, far smaller than any one would have anticipated. And
as their outline does not differ in any marked degree from that of all
other valleys, this conclusion may be extended to other cases.

In Glen Roy, where the three shelves can be seen near
each other, little or no difference can be perceived in their state of
preservation; indeed the upper one, I think, is more perfect than the
one below it. From this fact an argument has been advanced by Dr.
MACCULLOCH, that no long interval of time could have elapsed between
their formation. But this view is quite inadmissible; either the worn
and deeply notched rock of the shelves on Tombhran, or the buttresses
on the middle shelf (as at the head of Lower Glen Roy), which are
composed of large masses of well-rounded shingle, is sufficient,
without considering the intermediate shelf and other
appearances, to prove that the water must have remained at
levels intermediate between the highest and the 972-feet shelf for very
long periods. Hence the alternative is obvious, and is in direct
accordance with what has already been advanced, namely, that the
ordinary alluvial action is so exceedingly small, that whether the
surface has been exposed during one, two or more whole epochs, no
sensible difference can be perceived in the state of its conservation.

Of the many remarkable features in the geology of this
district, few, perhaps, are more remarkable than this perfect
preservation of its surface. We have a mound composed of soft materials
so small, that it oftentimes cannot be distinguished, by a person
standing on it, from the adjoining slope, but which it is not probable,
from the structure of the mountains, was ever much larger; and yet this
very mound, when viewed from a distance, will be seen to extend for
many hundred yards, even miles, continuous and perfect, with the
exception, perhaps, of a few small breaks, where some streamlet
descends. On these same mounds we can sometimes distinguish those
fragments which have been washed by the little waves of the ancient
waters, from others which have since fallen; and at Loch Treig, at the
height of 972 feet above the sea, the tide-scooped rocks appear as if
scarcely a century had elapsed since they were washed by the ripple of
the eddying currents. The preservation of the druidical mounds in
Britain has often been adduced as a circumstance worthy of attention;

but here during a period which cannot be reckoned by
thousands of years, but only by those great revolutions of nature which
are the effects of slow and scarcely sensible changes, works smaller
than those ancient ones dedicated to superstition, retain each outline
nearly as perfect as when first formed by the hand of nature.

These facts are interesting under another point of
view, for they prove to us that we may trust the plain inference of our
experiences. Although we see* the stone of many ancient buildings
decaying and crumbling away, yet we know that others, as the obelisks
of Egypt, have lasted more than three thousand years, with the
hieroglyphics nearly perfect on them: now we cannot see any reason why
their general outline, even in points of detail, should not last a
hundred times three thousand years. Again, although we might expect the
crest of a mountain range to be shattered, and the bed of a torrent to
be worn down more or less deeply, yet if we look at a convex slope of
soil clothed with turf, and drained on each side by rivulets, we can
see no reason, as long as the vegetation is persistent, why such a
slope (with the exception of any spot where a waterspout might burst,
or a stroke of lightning fall) should not last for as many thousand
centuries as the obelisks of Egypt shall remain entire. Of the justice
of these inferences, conclusive evidence is afforded by the state in
which we now see the mountains of Lochaber,—a state of which we
approximately know the high antiquity.

Section IX.—On the horizontality of the shelves,
and on the equable action of the elevatory forces.

Sir LAUDER DICK, with Mr. MACLEAN'S assistance, seems
to have determined within very small limits the absolute horizontality
of the several shelves. A delicate eighteen-inch levelling instrument,
made by JONES,1 was employed. Sir LAUDER says†, "Directing the
object-glass of the instrument to the nearer, and immediately opposite
corresponding line of shelf, it applied all along most accurately to
the horizontal hair; but when pointed to those further off (some of
which were perhaps five or six miles distant), they appeared to sink
sensibly below the hair, and this in proportion to their distance from
the point where we stood; but they were nowhere observed to do so in a
greater ratio than the allowance for the curvature of the earth at such
rectilineal distances demanded. And, what was in our opinion most
conclusive, when the telescope was pointed to, and made to traverse
along any particular portion, which, from being directly opposite to
the eye, might have been presumed to be nearly equidistant in all its
parts, it was found to preserve an uniform relation to the horizontal
hair." The same results were obtained in other instances; but yet the
angle of depression of the distant shelves does not appear to have been
actually measured, and its correspondence with the curve of the earth
calculated. But it is quite certain that if any

difference from that curve exists, it must be very
small*. Here then is a case which supports apparently with more weight
than perhaps any one hitherto advanced, the doctrine that the land is
the stationary element in these changes of level, and the ocean the
fluctuating one; for it may well be asked, can we suppose that a whole
country shall have been lifted up without the smallest ascertained
flexure of the ancient coast lines? Without reverting to the argument
of the movements now in progress, some upwards and some downwards, or
to the difficulty of imagining a receptacle for a stratum of water,
nearly 1300 feet thick, concentric with the globe, I will consider the
phenomenon in another point of view. It appears from the facts given by
Mr. LYELL in his Principles of Geology†, and in the Philosophical
Transactions‡, that a large territory in Sweden is now rising at the
rate of three feet in a century; and that the area affected reaches
from Gottenburgh to Torneo, and thence to North Cape (a distance of
1000 geographical miles), although the rate of elevation increases as
we proceed northward. We may therefore safely conclude, that large
spaces in Scandinavia have been elevated so equably, that at points
several miles, if not leagues apart, the difference of elevation at the
close of the past century, did not amount to one foot. In South America
the whole coast of Chile has been elevated within the recent period;
and during the great convulsions which affect that country, large
spaces have been uplifted nearly to the same amount, although some
parts a few feet more than others. On the eastern side of the same
continent, the land has also risen within the same period, and as
earthquakes are unknown there, the change probably has been, as in
Sweden, so slow as to be insensible at any one time. On that side the
traveller may ride for many hundred miles over plains, scarcely broken
by a single undulation, and where the strata and surface are almost
absolutely level: no one would there for one moment imagine that the
elevatory forces had acted unequally, but rather he is astonished that
the bottom of any sea or estuary should have been so uniform, as must
have been that of which the plains of La Plata not long since formed
the bed.

If then great plains and mountainous countries can be
raised within such small limits of absolute horizontality, as
undoubtedly has happened in the above cases, shall we, who are wholly
ignorant of the mechanism of these movements, be justified in rejecting
the plainest analogies, in supposing difficulties little short of
physical impossibilities, and in believing that the reverse of what is
ascertained in other cases has taken place in Lochaber, and all simply
because the change of level has been

* I may here remark, that the equal
elevation of the west coast of Scotland, and indeed of the whole
British Islands and other parts of Europe, may be inferred from the
facts collected by Mr. SMITH in his paper in the Edinburgh New
Philosophical Journal. This author says (vol. xxv. p. 388.), "The great
terrace (known to be of marine origin from the presence of organic
remains), the base of which seems very generally to be
between thirty and forty feet above the sea, forms a marked
feature in the scenery of the west of Scotland."

more equable, than we in our ignorance could have
anticipated? Every one, I think, who will attentively consider the
above facts, will answer with me in the negative, marvellous though the
fact be, that the beaches of Lochaber, raised on high so many hundred
feet, should still follow the curvature of their ancient waters. On the
contrary, a most important geological fact is established; namely that
an area (twenty miles in length and eighteen broad, and perhaps more,
if the shelves on the banks of the Spey be included in it) has been
raised 1278 feet above the level of the sea, so equably, that no
deviation from the true curvature of the earth can be discovered by the
ordinary means of leveling*.

Section X.—Speculations on the action of the
elevatory forces, and conclusion.

If we choose to enter on speculative grounds and to
reflect on the secondary means which have caused these equable
movements, two solutions occur. But first I must remark that the crust
of the earth seems to yield easily to the forces which have acted on it
from below; when we observe a brick-wall dashed to pieces by a cannon
ball, or a pane of glass by a small stone, we say that both are fragile
and yield easily; so when we examine the earth and find it fissured and
refissured, one fragment let down and another raised high up (as we
know to be the case where extensive sections have been obtained, as in
our coal-pits or metalliferous districts), we must certainly admit,
that the force which has broken up the crust in vertical planes
relatively nearer to each other, compared with its thickness, than in
the fissured pane of glass, easily overcame the resistance offered to
it, however absolutely great that may have been. This same conclusion
is forced on us, when we reflect that the very cause of the trembling
of the ground in earthquakes seems due to the rending of the strata;
and that earthquakes in many countries are of such frequent occurrence,
that probably this hour will scarcely elapse without the crust
somewhere yielding. If indeed the crust did not yield readily, partial
elevations could not be so gradual as they are known to be, but they
would assume the character of explosions. That there has been some real
connexion in certain cases† between that state of the weather which is
accompanied by a low barometer and the occurrence of earthquakes, can,
I think, hardly be doubted; if we admit Mr. P. SCROPE'S1 explanation of
this, that the diminution of atmospheric pressure (equal in some cases
to an inch and half of mercury, spread over a very large area)
determines the particular time at which the earthquake occurs, the
force and tension being before almost balanced, we may be said to
possess a rude measure of the force requisite in that area to overcome
the coherence of the parts, as existing in the intervals of the
recurrent earthquakes. If then the mo-

* Considering the great importance of
this conclusion, and the many points of interest connected with the
subject of the 'parallel roads' it is greatly to be desired that the
admirable opportunity for a close examination, afforded by the intended
Ordnance Survey, will be taken advantage of by the gentlemen, so well
qualified for the task, who conduct it.

† In my Journal during the voyage of
the
Beagle, I have mentioned (p. 431 and 432.) some instances of this.

1 George Julius Poulett-Scrope (1797-1876), geologist and political economist.

tive force acts so gradually that the earth's crust can
acquire that degree of tension, which causes large portions of it to
yield readily to a very slight additional impulse; and if, as we know
undoubtedly to be the case, the crust has yielded in innumerable
vertical planes, intersecting each other like a net-work, and running
parallel to each other at very short distances, we are compelled to
admit that the equable elevation of so large an extent of country as
Lochaber, must have resulted from the equable action of the elevatory
forces, and not from the cohesion of its parts.

Bearing this in mind, the most obvious solution, but I
very much doubt whether the correct one, is, that no force excepting
the uniform expansion of solid matter from heat, could raise so equably
the surface of a great fragile mass, as the district of
Lochaber must be considered. I doubt this solution, first, because a
very great expansion is necessary, especially if we include in these
movements the elevation of the erratic blocks, now lying more than 2200
feet above the sea. Secondly, because the movements appear to have been
of the same kind as those in the not distant country of Sweden; and
there it has been shown by Mr. LYELL, that near Stockholm an alternate
movement of more than sixty feet has taken place within the human
period; and one is strongly tempted to believe that there is some
relative connexion between the areas in Northern Europe which are
rising and those which are quietly subsiding. These facts to be
explicable on the theory of expansion, require, as it appears to me,
far too capricious an action, in so slowly and far-pervading
an influence as heat, to be admitted; whilst on the supposition of
mechanical displacement such difficulties are not presented. Thirdly,
because (and it is my chief reason for rejecting the agency of
expansion by itself) the movements appear to have been of the same
order with those now in progress in South America; and in that country
the elevation of certain wide areas, as I endeavoured to show in a
paper lately (March 7, 1838) read before the Geological Society,1 cannot
be attributed to any other cause than an actual movement in
the subterranean expanse of molten rock: to speak only for example
sake, such as would result from a change in position of those
inequalities in the ellipticity of the earth's surface, which seem
indicated by the measurements of arcs of meridians. It may also be
inferred, from the facts given in that paper, that the fluidity of the
nucleus must be tolerably perfect. In the volcano, even the lava which
is propelled to the summit of a mountain, far beyond the subterranean
isothermal line of melted rock, and poured out on the surface, is
oftentimes so fluid, that it runs into thin sheets like molten metal.
Also at the junction of the plutonic with the metamorphic formations,
we see tortuous thread-like veins branching from the former into the
latter, which could only have been injected when quite liquid. Here the
rock has been melted at a great depth under an enormous pressure, and
yet the fluidity must have been very perfect: such plutonic rocks
moreover form the beds on which all others rest. Considering these
latter facts, together with the inferences deduced from the phenomena
observed in South America, it may be granted as not improbable in any
high degree, that this part of Scotland when it was upraised rested

on matter possessed of considerable fluidity, which
underwent a slow change of form. If this be granted, there is no great
difficulty in conceiving that the surface of the interior molten matter
might retain that degree of curvature proper to it, as the resultant of
the unknown force with that of gravity and the centrifugal impulse.
Moreover, as we must conclude from what we now see going on in South
America and in Scandinavia, that the area affected was large, the
difference between the amount of curvature of the fluid nucleus after
the rise in that part of one or two thousand feet, would be exceedingly
small, and its outline scarcely distinguishable from that of the ocean,
and certainly not from that of a sea affected by various tides in
confined channels, which in the case of Glen Roy affords the only
standard of comparison. We may almost venture to say, that as the
packed ice on the Polar Sea, with its hummocks and wide floes, rises
over the tidal wave, so did the earth's crust with its mountains and
plains rise on the convex surfaces of molten rock, under the influence
of the great secular changes then in progress.

After these considerations I am far from thinking it an
overwhelming difficulty, that the curvature of the shelves of Glen Roy
over a space of four, or five, or perhaps even twenty miles should
appear to be the same with that of the surface of the ocean, within
that limit of accuracy which the nature of the case renders possible.
On the contrary, I deduce from their curvature, first, that the
district of Lochaber formed only a small part of the area affected;
secondly, a confirmation of the view, which I deduced from the
phenomena observed in South America, that the motive power in such
cases is a slight additional convexity slowly added to the fluid
nucleus; and thirdly, this additional fact, that we thus obtain some
measure of the degree of homogeneous fluidity of the subterranean
matter beneath a large area, namely, that its particles, when acted on
by a disturbing force, arrange themselves in obedience to the law of
gravity. And although we arrive at this conclusion with some surprise,
when relating to the abysses of the nether regions, we see it
habitually verified in volcanic countries, where a torrent of lava,
checked by some obstacle, has expanded into a level sheet.

Mr. LYELL, in his Principles of Geology*, quotes a
passage from Sir JOHN HERSCHEL'S1 Astronomy†, to show that whatever may
have been the original figure of the earth, the wearing down of the
solid matter and its redeposition at the bottom of the sea, must tend
continually to change the actual figure of the earth, as
PLAYFAIR‡ expresses it, into the statical one: he then adds,
"that the same remark applies to every stream of lava flowing on the
surface, and if the volcanic action should extend to great depths, so
as to melt one after another different parts of the earth, the whole
interior might at length be remodeled under the influence of similar
changes, due to causes which may all be operating at this moment." Now
if it be granted that the curvature

of the shelves of Lochaber is due to the elevation of
the district by means of a subterraneous expanse of fluid matter; the
atoms of which obeyed the law of gravity, it cannot be doubted they
would likewise obey that of the centrifugal force. Therefore, if the
figure of the earth did not already very nearly approach to that of a
spheroid of equilibrium, regions near the equator and others near the
poles, during the changes of level now actually in progress, would be
acted on by forces greatly different; and consequently as the crust
does now yield (and has yielded in an infinite number of planes,) the
statical form would be immediately acquired. This view is here given,
because a directly opposite, and as I cannot but think incorrect one,
has been advanced by PLAYFAIR*.

In concluding this paper, I will briefly indicate the
chief points which receive illustration from the examination of the
district of Lochaber by Sir THOMAS LAUDER DICK, Dr. MACCULLOCH, and
myself. 1st. Nearly the whole of the waterworn materials in the valleys
of this part of Scotland were left, as they now occur, by the slowly
retiring waters of the sea; and the chief action of the rivers since
that period has been to remove such deposits; and when this was
effected, to excavate a wall-sided gorge in the solid rock. 2nd. During
the vast period which must have elapsed since the sea stood at the
level of the upper shelves, the alluvial action has been exceedingly
small: steep slopes of turf over large spaces and the bare surface of
rocks have been preserved even perfectly; and we see every main, as
well as most of the lesser inequalities of the land, in the state in
which they were then left. 3rd. The elevation of this part of Scotland
from the level of the present beach to the height of at least 1278
feet has been extremely gradual, and was interrupted by long intervals
of rest: it has taken place since the so-called "erratic block period."
4th. It is probable that the erratic blocks were transported during the
quiet formation of the shelves. One was observed to occur at an
altitude of 2200 feet above the level of the sea. 5th. The
extraordinary fact that a large country has been elevated to a great
height so equably, that the ancient beach-lines retain the same, or
nearly the same curvature, which they had when bounding the convex
surface of the ancient waters. Lastly. The inferences from this head,
supported by other cases, namely, that a large area must have been
upraised, and that this was effected by a slight change in the convex
form of the fluid matter on which the crust rests; and, therefore, that
the fluidity is sufficiently perfect to allow of the atoms moving in
obedience to the law of gravity, and consequently of the effects of
that law modified by the centrifugal impulse. Hence, that even the
disturbing forces do not tend to give to the earth a figure widely
different from that of a spheroid in equilibrium.

POSTSCRIPT.

I am much indebted to my friend Mr. ALBERT WAY1 for his
kindness in lending me the drawing, from which the accompanying
lithographic sketch has been taken. It very faithfully represents the
general appearance of Glen Roy.

* Illustrations of the Huttonian Theory,
p. 488.

1 Albert Way (1805-1874), antiquary and traveller, a friend of Darwin's from their student days in Cambridge. Way sketched the well-known student portaits of Darwin astride beetles.